powerpc: Fix multicast problem in fs_enet driver

mac-fec.c was setting individual UDP address registers instead of multicast
group address registers when joining a multicast group.
This prevented from correctly receiving UDP multicast packets.
According to datasheet, replaced hash_table_high and hash_table_low
with grp_hash_table_high and grp_hash_table_low respectively.
Also renamed hash_table_* with grp_hash_table_* in struct fec declaration
for 8xx: these registers are used only for multicast there.

Tested on a MPC5121 based board.
Build tested also against mpc866_ads_defconfig.

Signed-off-by: Andrea Galbusera <gizero@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>