drivers: net: emac: add const to mii_phy_ops structures

The object references of mii_phy_ops structures are only stored
in the ops field of a mii_phy_def structure. This ops field is of type
const. So, mii_phy_ops structures having similar properties can be
declared as const.

Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>