qmi_wwan_q: kernel 6.18 compatibility fixes

[ozgunokan]

qmi_wwan_q: kernel 6.18 compatibility fixes

• hrtimer_init() -> hrtimer_setup() (removed upstream in kernel 6.11+)
• usbnet_bh() tasklet hijack removed: usbnet replaced the tasklet-based
  bh with a private workqueue (bh_work) that external drivers can no
  longer intercept
• qmap_wake_queue() call moved from the removed usbnet_bh() hook into
  rmnet_usb_rx_fixup(), which usbnet already calls on every completed
  RX -- equivalent timing to the original hook

Tested on IPQ807x (Arcadyan AW1000, Quectel RG500Q-EA), kernel 6.18.44,
NSS-accelerated rmnet confirmed via qca-nss-drv rmnet_rx counters.

[FUjr]

Thank you for the kernel 6.18 work and the Turkish SMS contribution. I reviewed both commits, but I cannot merge the PR as-is because it combines unrelated changes and has compatibility/regression risks:

• hrtimer_setup() is used unconditionally even though this driver still supports older kernel branches where that API is unavailable; it needs an appropriate version guard and the existing fallback.
• Moving qmap_wake_queue() exclusively to the RX fixup is not equivalent to the previous bottom-half timing. Upload-only traffic can leave child QMAP queues stopped until an RX packet arrives.
• The Turkish GSM 7-bit mappings are applied globally. National locking/single-shift tables must be selected from the relevant UDH language IE; otherwise non-Turkish messages using reserved extension codes can be decoded differently.
• The SMS parser still accepts an unbounded, unvalidated modem-provided PDU into a fixed 512-byte buffer.

Please split the kernel and SMS changes into separate PRs and address these points. Thanks again for testing this on IPQ807x/RG500Q-EA and for taking the time to contribute.

[sqliuchang]

On the IPQ807x target with kernel 6.18, only the generic QMI and MHI drivers currently build without issues—both vendor and NSS variants fail to compile.
I've verified that this PR successfully fixes the build for the NSS-accelerated QMI driver.

[FUjr]

I have rebuilt this branch on the latest main and separated it from the SMS work.

The NSS changes now include:

• automatic hrtimer_init()/hrtimer_setup() selection by kernel version;
• restoration of the original usbnet bottom-half hook on older kernels, without the unsafe global work-function hijack;
• an opt-in QMODEM_QMI_NSS_RX_WAKE_FALLBACK package build option for the newer-kernel RX-side queue wake workaround (default: disabled);
• an opt-in QMODEM_QMI_NSS_REGISTRATION_RETRY package build option for transient NSS registration failures (default: disabled);
• checked RX-handler registration with cleanup on failure, bounded retry scheduling, and synchronous cancellation during teardown.

The updated head is 78f373e. This PR is now NSS/kernel-only; the Turkish SMS changes remain in PR #259.

The two behavioral workarounds are intentionally compile-time options so a target can enable them only after validating its vendor kernel/NSS integration. Would you be comfortable merging this split and gated version?