wl1251: implement multicast address filtering (fwd)

Port multicast address filtering from wl1271 driver.
It sets up the hardware multicast address filter in configure_filter() with
addresses supplied through prepare_multicast().

Signed-off-by: David Gnedt <david.gnedt@davizone.at>
Signed-off-by: Pali Rohár <pali.rohar@gmail.com>
Signed-off-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: John W. Linville <linville@tuxdriver.com>