Add ethtool PAM4 link modes for:
        50000baseCR_Full
        100000baseCR2_Full
        200000baseCR4_Full
Signed-off-by: Edwin Peer <edwin.peer@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>