r6040: fix multicast operations

The original code does not work well when the number of mulitcast
address to handle is greater than MCAST_MAX. It only enable promiscous
mode instead of multicast hash table mode, so the hash table function
will not be activated and all multicast frames will be recieved in this
condition.

This patch fixes the following issues with the r6040 NIC operating in
multicast:

1) When the IFF_ALLMULTI flag is set, we should write 0xffff to the NIC
hash table registers to make it process multicast traffic.

2) When the number of multicast address to handle is smaller than
MCAST_MAX, we should use the NIC multicast registers MID1_{L,M,H}.

3) The hashing of the address was not correct, due to an invalid
substraction (15 - (crc & 0x0f)) instead of (crc & 0x0f) and an
incorrect crc algorithm (ether_crc_le) instead of (ether_crc).

4) If necessary, we should set HASH_EN flag in MCR0 to enable multicast
hash table function.

Reported-by: Marc Leclerc <marc-leclerc@signaturealpha.com>
Tested-by: Marc Leclerc <marc-leclerc@signaturealpha.com>
Signed-off-by: Shawn Lin <shawn@dmp.com.tw>
Signed-off-by: Albert Chen <albert.chen@rdc.com.tw>
Signed-off-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: David S. Miller <davem@davemloft.net>