net: enetc: build common object files into a separate module

The build system is complaining about the following:

enetc.o is added to multiple modules: fsl-enetc fsl-enetc-vf
enetc_cbdr.o is added to multiple modules: fsl-enetc fsl-enetc-vf
enetc_ethtool.o is added to multiple modules: fsl-enetc fsl-enetc-vf

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>