Add a dedicated function to toggle port link. It should be called only
after setting a port register.
Toggle will set port link to down and bring it back up in case that it's
admin status was up.

Signed-off-by: Gal Pressman <galp@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>