net/mlx5e: Fix 50G per lane indication

Some released FW versions mistakenly don't set the capability that 50G
per lane link-modes are supported for VFs (ptys_extended_ethernet
capability bit). When the capability is unset, read
PTYS.ext_eth_proto_capability (always reliable).
If PTYS.ext_eth_proto_capability is valid (has a non-zero value)
conclude that the HCA supports 50G per lane. Otherwise, conclude that
the HCA doesn't support 50G per lane.

Fixes: a08b4ed1 ("net/mlx5: Add support to ext_* fields introduced in Port Type and Speed register")
Signed-off-by: Aya Levin <ayal@mellanox.com>
Reviewed-by: Eran Ben Elisha <eranbe@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>