fix promiscuous mode for sunxi_emac driver

The sunxi-emac driver is rather primitive, and doesn't support
promiscuous mode. This is a problem when you try do to configure
a bridge, or capturing ethernet traffic.

Based on a patch for linux mainline from Marc Zyngier: 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.

Tested on a20-olinuxino-micro.

Signed-off-by: Diego Roversi <diegor@tiscali.it>