davinci_emac.c: Fix IFF_ALLMULTI setup

When IFF_ALLMULTI flag is set on interface and IFF_PROMISC isn't,
emac_dev_mcast_set should only enable RX of multicasts and reset
MACHASH registers.

It does this, but afterwards it either sets up multicast MACs
filtering or disables RX of multicasts and resets MACHASH registers
again, rendering IFF_ALLMULTI flag useless.

This patch fixes emac_dev_mcast_set, so that multicast MACs filtering and
disabling of RX of multicasts are skipped when IFF_ALLMULTI flag is set.

Tested with kernel 2.6.37.

Signed-off-by: Mariusz Ceier <mceier+kernel@gmail.com>
Acked-by: Mugunthan V N <mugunthanvnm@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>