dahdi-linux: forward-port to 6.18 kernel APIs, fix build flags, disable OSLEC

[graysky2]

Maintainer: no one
Compile tested: (put here arch, model, OpenWrt version) SNAPSHOT
Run tested: (put here arch, model, OpenWrt version, tests done) not run tested

Description:

This patchset updates DAHDI 3.4.0 to build cleanly on Linux 6.18 and modern OpenWrt toolchains. Changes include:

• Add kernel API compatibility shims:

  • Provide hrtimer_init() wrapper using hrtimer_setup()
  • Restore from_timer() helper
  • Map del_timer*() to timer_delete*() on >= 6.15, aligning with upstream PR net/asterisk: Update to Asterisk 11.20.0 and 13.6.0 #93[1].

• Replace deprecated EXTRA_CFLAGS with ccflags-y across all Kbuilds to match upstream kernel changes and resolve build failures. Relating to oct612x include paths using ccflags-y, aligning with upstream PR sipp: fix musl compatibility #76[2]
  .

• Disable OSLEC echo canceller:

  • OpenWrt provides only oslec.h without echo.c implementation
  • Prevents undefined oslec_* symbol failures during modpost
  • Marks dahdi-echocan-oslec package as unavailable on >= 6.18

• Minor Makefile adjustments for OpenWrt packaging consistency.

These changes collectively restore a complete, warning-free build of DAHDI 3.4.0 on Linux 6.18 while preserving compatibility with existing drivers and OpenWrt module packaging.

1. dahdi_dummy: Build failure with kernel >= 6.15 asterisk/dahdi-linux#93
2. oct612x: Build failure due to EXTRA_CFLAGS being removed in the kernel asterisk/dahdi-linux#76

[dangowrt]

OSLEC used to work fine with older kernels, it is built as module kmod-echo. It would be nice if don't have to drop it, I'm actively using it with the HFC-S BRI driver (HFC-S is a low-cost BRI chip without any DSP functionality, without OSLEC there is no echo cancelation at all, which is the more annoying the more latency there is in a call)