brcmsmac: Add missing annotation for brcms_rfkill_set_hw_state()

Sparse reports a warning at brcms_rfkill_set_hw_state()

warning: context imbalance in brcms_rfkill_set_hw_state()
	- unexpected unlock
The root cause is the missing annotation at brcms_rfkill_set_hw_state()
Add the missing __must_hold(&wl->lock) annotation

Signed-off-by: Jules Irenge <jbi.octave@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200411001933.10072-5-jbi.octave@gmail.com