net: dsa: mv88e6xxx: Save power by disabling SerDes trasmitter and receiver

Save power on 88E6393X by disabling SerDes receiver and transmitter
after SerDes is SerDes is disabled.

Signed-off-by: Marek Behún <kabel@kernel.org>
Cc: stable@vger.kernel.org # de776d0d ("net: dsa: mv88e6xxx: add support for mv88e6393x family")
Signed-off-by: David S. Miller <davem@davemloft.net>