net: fsl/fman: rename IF_MODE_XGMII to IF_MODE_10G

As the only 10G PHY interface type defined at the moment the code
was developed was XGMII, although the PHY interface mode used was
not XGMII, XGMII was used in the code to denote 10G. This patch
renames the 10G interface mode to remove the ambiguity.

Signed-off-by: Madalin Bucur <madalin.bucur@oss.nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>