mv88e6xxx: Combine mv88e6131 and mv88e612_61_65 drivers

These drivers share a lot of code, so if we make them modular they
should be built into the same module.  Therefore, link them together
and merge their respective module init and exit functions.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>