brcmsmac: Add missing annotation for brcms_down()

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

drivers/net/wireless/broadcom/brcm80211/brcmsmac/mac80211_if.c

@@ -1431,6 +1431,7 @@ int brcms_up(struct brcms_info *wl)
  * precondition: perimeter lock has been acquired
  */
 void brcms_down(struct brcms_info *wl)
+	__must_hold(&wl->lock)
 {
 	uint callbacks, ret_val = 0;