brcmsmac: allow user-space setting of interface address

The interface address of the wireless device is determined by
the permanent address stored in the device. This patch allows
it to be overridden from user-space.

Reported-by: Maximilian Engelhardt <maxi@daemonizer.de>
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>