drivers: net: ldpaa_eth: check if the dpmac is enabled

drivers: net: ldpaa_eth: check if the dpmac is enabled

some dpmacs in armv8a based freescale layerscape SOCs can be
configured via both serdes(sgmii, xfi, xlaui etc) bits and via
EC*_PMUX(rgmii) bits in RCW.
e.g. dpmac 17 and 18 in LX2160A can be configured as SGMII from
serdes bits and as RGMII via EC1_PMUX/EC2_PMUX bits
Now if a dpmac is enabled by serdes bits then it takes precedence
over EC*_PMUX bits. i.e. in LX2160A if we select serdes protocol
that configures dpmac17 as SGMII and set the EC1_PMUX as RGMII,
then the dpmac is SGMII and not RGMII.

Therefore, in fsl_rgmii_init function of SOC, we will check if the
dpmac is enabled or not? if it is (fsl_serdes_init has already enabled
the dpmac), then don't enable it.

Signed-off-by: Pankaj Bansal <pankaj.bansal@nxp.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>
Reviewed-by: Prabhakar Kushwaha <prabhakar.kushwaha@nxp.com>