igc: Remove unused MII_CR_LOOPBACK

MII_CR_LOOPBACK masks not in use in i225 device and can be removed.

Signed-off-by: Sasha Neftin <sasha.neftin@intel.com>
Tested-by: Dvora Fuxbrumer <dvorax.fuxbrumer@linux.intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>