net: sun4i-emac: add promiscuous support

The sun4i-emac driver is rather primitive, and doesn't support
promiscuous mode. This makes usage such as bridging impossible,
which is a shame on virtualization capable HW such as the
Allwinner A20.

The fix is fairly simple: move the RX setup code to the ndo_set_rx_mode
vector, and add the required HW configuration when IFF_PROMISC is passed
by the core code.

This has been tested on a generic A20 box running a few virtual
machines hanging off a bridge with the EMAC chip as the link to the
outside world.

Cc: Stefan Roese <sr@denx.de>
Cc: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Acked-by: Stefan Roese <sr@denx.de>
Signed-off-by: David S. Miller <davem@davemloft.net>