net: lx2160a.c: Update to set ECx_PMUX precedence

As per hardware documentation, ECx_PMUX has precedence
over SerDes protocol.
For LX2160/LX2162 if DPMACs 17 and 18 are enabled as SGMII
through SerDes protocol but ECx_PMUX configured them as RGMII,
then the ports will be configured as RGMII and not SGMII.

Signed-off-by: Razvan Ionut Cirjan <razvanionut.cirjan@nxp.com>
[Rebased]
Signed-off-by: Priyanka Jain <priyanka.jain@nxp.com>