r6040: fix r6040_multicast_list

As reported in <https://bugzilla.kernel.org/show_bug.cgi?id=15355

>, r6040_
multicast_list currently crashes. This is due a wrong maximum of multicast
entries. This patch fixes the following issues with multicast:

- number of maximum entries if off-by-one (4 instead of 3)

- the writing of the hash table index is not necessary and leads to invalid
values being written into the MCR1 register, so the MAC is simply put in a non
coherent state

- when we exceed the maximum number of mutlticast address, writing the
broadcast address should be done in registers MID_1{L,M,H} instead of
MID_O{L,M,H}, otherwise we would loose the adapter's MAC address

Signed-off-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: David S. Miller <davem@davemloft.net>