net: bgmac: make it clear when setting interface type to RMII

It doesn't really change anything as BGMAC_CHIPCTL_1_IF_TYPE_RMII is
equal to 0. It make code a bit clener, so far when reading it one could
think we forgot to set a proper mode. It also keeps this mode code in
sync with other ones.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: David S. Miller <davem@davemloft.net>