drivers: net: fsl_enetc: fix SXGMII MAC configuration

Separate MAC and serdes configuration, MAC configuration must be applied
at each enetc_start() as FLR clears it.
This restores traffic for ENETC interfaces in USXGMII mode.

Signed-off-by: Alex Marginean <alexandru.marginean@nxp.com>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>