mwl8k: fix multicast address filter programming

mwl8k's ->prepare_multicast() currently just enables reception of
all multicast packets, which is somewhat ineffective.

Fix this by either disabling all multicast RX, enabling multicast
RX according to the multicast address filter table, or enabling all
multicast RX, depending on whether ->prepare_multicast() was given
any multicast addresses and whether the hardware multicast address
filter table is large enough to fit all requested addresses.

Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>