gianfar: Restore promisc mode on gfar_init_mac()

Reactivate promiscuous mode in H/W upon gfar_init_mac(), if the
net dev requires it (IFF_PROMISC flag set).
This way the promisc mode is preserved accross device reset conditions
like tx timeout, device restore, a.s.o.

Signed-off-by: Voncken C Acksys <cedric.voncken@acksys.fr>
Signed-off-by: Claudiu Manoil <claudiu.manoil@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>