net: dsa: mv88e6xxx: enable SERDES after setup

SERDES is powered on for CPU and DSA ports and powered down for unused
ports at setup time. But now that DSA calls mv88e6xxx_port_enable
and mv88e6xxx_port_disable for all ports, the SERDES power can now
be handled after setup inconditionally for all ports.

Using the port enable and disable callbacks also have the benefit to
handle the SERDES IRQ for non user ports as well.

Signed-off-by: Vivien Didelot <vivien.didelot@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>