net: dsa: mv88e6xxx: wrap .set_egress_port method

There are two implementations of the .set_egress_port method, and both
of them, if successful, set chip->*gress_dest_port variable.

To avoid code repetition, wrap this method into
mv88e6xxx_set_egress_port.

Signed-off-by: Marek Behún <kabel@kernel.org>
Reviewed-by: Pavana Sharma <pavana.sharma@digi.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>