net/mlx5: Lift reload limitation when SFs are present

Historically, the shared devlink_mutex prevented devlink instances from
being registered/unregistered during another devlink instance reload
operation. However, devlink_muxex is gone for some time now, this
limitation is no longer needed. Lift it.

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