assume RGMII-to-Copper mode by default in eephy(4) for 88E151x PHYs

assume RGMII-to-Copper mode by default in eephy(4) for 88E151x PHYs

The desired MII mode must be programmed explicitly for Marvel Atlantis
88E1512/88E1514 variants and we already do this for SGMII.

This change adds a missing case for RGMII-to-Copper that assumes RGMII,
unless the MAC driver sets MII_SGMII before calling mii_attach() or the
mode has already been programmed. (RGMII-to-Copper is also the hardware
default for E1510 and E1518.)

Suggestions and ok kettenis@