brcmfmac: fix missing unlock on error in brcmf_notify_vif_event()

Add the missing unlock before return from function brcmf_notify_vif_event()
in the error handling case.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>