Sparse reports a warning at brcms_down()

warning: context imbalance in brcms_down()
	- unexpected unlock
The root cause is the missing annotation at brcms_down()
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/20200411001933.10072-6-jbi.octave@gmail.com