mac80211: fix build for 6.12.64 bump

mac80211: fix build for 6.12.64 bump

Disable the backport shim on 6.12.64 and newer where the function
is provided by the kernel.

References:
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/include/linux/hrtimer.h?h=v6.12.64&id=12493e7e888d266453d1c3898dc160178749afb3
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/include/linux/hrtimer.h?h=v6.12.64&id=d579cc549298a02ba90b7a6d20a2ba8e160f3f31

Co-Authored-by: Pascal Ernster <git@hardfalcon.net>
Signed-off-by: John Audia <therealgraysky@proton.me>
Link: https://github.com/openwrt/openwrt/pull/21431
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>