brcmsmac: use perimeter lock in add_interface() callback

All callbacks that access driver functions should do that under
perimeter lock protection. The add_interface() callback was lacking
this lock.

Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Reviewed-by: Piotr Haber <phaber@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>