enetc: Clean up makefile

Clean up overcomplicated makefile to make it more maintainable.
Basically, there's a set of common objects shared between
the PF and VF driver modules.  This can be implemented in a
simpler way, without conditionals, less repetition, allowing
also for easier updates in the future.

Signed-off-by: Claudiu Manoil <claudiu.manoil@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>