selftests: mlxsw: Check device info on activated line card

Once line card is activated, check the device FW version is exposed.

Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>