net/mlx4_en: Use PTYS register to set ethtool settings (Speed)

Added Support to set speed or advertised link modes via ethtool:
ethtool -s <ifname> [speed <speed>] [advertise <link modes>]

Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: Amir Vadai <amirv@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>