net/mlx5: Query hca_cap_2 only when supported

On vport enable, where fw's hca caps are queried, the driver queries
hca_caps_2 without checking if fw truly supports them, causing a false
failure of vfs vport load and blocking SRIOV enablement on old devices
such as CX4 where hca_caps_2 support is missing.

Thus, add a check for the said caps support before accessing them.

Fixes: e5b9642a ("net/mlx5: E-Switch, Implement devlink port function cmds to control migratable")
Signed-off-by: Maher Sanalla <msanalla@nvidia.com>
Reviewed-by: Shay Drory <shayd@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>