net: dsa: mv88e6xxx: Fix application of erratum 4.8 for 88E6393X

According to SERDES scripts for 88E6393X, erratum 4.8 has to be applied
every time before SerDes is powered on.

Split the code for erratum 4.8 into separate function and call it in
mv88e6393x_serdes_power().

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