net: dsa: mediatek: add adjust link support for user ports

Manually adjust the port settings of user ports once PHY polling has
completed. This patch extends the adjust_link callback to configure the
per port PMCR register, applying the proper values polled from the PHY.
Without this patch flow control was not always getting setup properly.

Signed-off-by: Shashidhar Lakkavalli <shashidhar.lakkavalli@openmesh.com>
Signed-off-by: Muciri Gatimu <muciri@openmesh.com>
Signed-off-by: John Crispin <john@phrozen.org>
Signed-off-by: David S. Miller <davem@davemloft.net>