Blackfin EMAC driver: add function to change the MAC address

Alex Landau writes in the forums:
 Previously, changing the MAC address (e.g. via ifconfig) resulted
 in a generic function to be called that only changed a variable in
 memory. This patch also updated the Blackfin MAC address registers
 to filter the correct new MAC.

Signed-off-by: Alex Landau <lirsb@yahoo.com>
Signed-off-by: Mike Frysinger <michael.frysinger@analog.com>
Signed-off-by: Bryan Wu <bryan.wu@analog.com>
Acked-by: Jeff Garzik <jeff@garzik.org>
Signed-off-by: Jeff Garzik <jeff@garzik.org>