net/mlxfw: fix a NULL dereference

If we hit this error path we end up returning ERR_PTR(0) which is NULL.
The caller is not expecting that so it results in a NULL dereference.

Fixes: 410ed13c ("Add the mlxfw module for Mellanox firmware flash process")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Yotam Gigi <yotamg@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>