dsa: lan9303: Add flow ctrl in link_up

While the prior patch moved the adjust_link code into the
phylink_mac_link_up api, this patch cleans it up and adds the setting the
port's flow control based on the phylink_mac_link_up input parameters.

Signed-off-by: Jerry Ray <jerry.ray@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>