IB/mlx5: Return error for unsupported signature type

In case of unsupported singature, we returned positive
value, while the better approach is to return -EINVAL.

In addition, in this change, the error print is enriched
to provide an actual supplied signature type.

Fixes: e6631814 ("IB/mlx5: Support IB_WR_REG_SIG_MR")
Cc: Sagi Grimberg <sagi@grimberg.me>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Doug Ledford <dledford@redhat.com>