wifi: rt2x00: Switch to use hrtimer_update_function()

The field 'function' of struct hrtimer should not be changed directly, as
the write is lockless and a concurrent timer expiry might end up using the
wrong function pointer.

Switch to use hrtimer_update_function() which also performs runtime checks
that it is safe to modify the callback.

Signed-off-by: Nam Cao <namcao@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/82dcc0eae40bb84e6452f242751c0650e79bd87e.1738746927.git.namcao@linutronix.de