net: ethernet: mediatek: fix RMII mode and add REVMII supported by GMAC

The patch fixes up the incorrect setup of reduced MII (RMII) on GMAC
and adds the supplement for the setup of reverse MII (REVMII) on GMAC
, and rearranges the error handling for invalid PHY argument.

Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: David S. Miller <davem@davemloft.net>