net/mlx5e: Fix ethtool self test: link speed

Ethtool self test contains a test for link speed. This test reads the
PTYS register and determines whether the current speed is valid or not.
Change current implementation to use the function mlx5e_port_linkspeed()
that does the same check and fails when speed is invalid. This code
redundancy lead to a bug when mlx5e_port_linkspeed() was updated with
expended speeds and the self test was not.

Fixes: 2c81bfd5

 ("net/mlx5e: Move port speed code from en_ethtool.c to en/port.c")
Signed-off-by: Aya Levin <ayal@mellanox.com>
Reviewed-by: Moshe Shemesh <moshe@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>