brcmfmac: Make probe function __init

One of the benefits of platform_driver_probe() is that you can make
the probe function __init.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Cc: Hante Meuleman <meuleman@broadcom.com>
Cc: Arend van Spriel <arend@broadcom.com>
Cc: John W. Linville <linville@tuxdriver.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>