ethtool: Add 50G baseSR2 link mode

Add ETHTOOL_LINK_MODE_50000baseSR2_Full_BIT bit.

Signed-off-by: Gal Pressman <galp@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Cc: Ben Hutchings <bwh@kernel.org>
Cc: David Decotigny <decot@googlers.com>
Acked-By: David Decotigny <decot@googlers.com>
Signed-off-by: David S. Miller <davem@davemloft.net>