net-next: mediatek: fix gigabit and flow control advertisement

The current code will not setup the PHYs advertisement features correctly.
Fix this and properly advertise Gigabit features and properly handle
asymmetric pause frames.

Signed-off-by: Sean Wang <keyhaede@gmail.com>
Signed-off-by: John Crispin <john@phrozen.org>
Signed-off-by: David S. Miller <davem@davemloft.net>