net: pch_gbe: Clean up pch_gbe_set_multi

Refactor pch_gbe_set_multi in order to avoid unnecessary indentation &
make it clearer what the code is doing.

The one behavioral change from this patch is that we'll no longer
configure the MAC address registers for multicast addresses when the
IFF_PROMISC or IFF_ALLMULTI flags are set. In these cases, just as when
we want to monitor more multicast addresses than we have MAC address
registers, we disable multicast filtering so the MAC address registers
are unused.

Signed-off-by: Paul Burton <paul.burton@mips.com>
Signed-off-by: David S. Miller <davem@davemloft.net>