net/mlx5: Fix uninitialized variable bug in outlen_write()

If sscanf() return 0, outlen is uninitialized and used in kzalloc(),
this is unexpected. We should return -EINVAL if the string is invalid.

Fixes: e126ba97 ("mlx5: Add driver for Mellanox Connect-IB adapters")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>