mlxfw: Make the module selectable

There are upcoming NIC (mlx5) use-cases where people want to avoid
building the mlxfw module, allow for that. The mlxsw module is
untouched and keeps selecting mlxfw.

Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Acked-by: Yotam Gigi <yotamg@mellanox.com>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>