mv88e6xxx: Use cached config3 in 6352 has_serdes

mv88e6xxx: Use cached config3 in 6352 has_serdes

1. Refactor mv88e6352_g2_scratch_port_has_serdes to use the cached
scratch config3 value instead of reading it everytime.
2. Remove err<0 check from mv88e6352_phylink_get_caps as it is never
true anymore

Co-developed-by: Thomas Eckerman <thomas.eckerman.ext@ericsson.com>
Signed-off-by: Thomas Eckerman <thomas.eckerman.ext@ericsson.com>
Signed-off-by: Fidan Aliyeva <fidan.aliyeva.ext@ericsson.com>
Link: https://patch.msgid.link/20260521202924.727929-4-fidan.aliyeva.ext@ericsson.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>