net/mlx5: Fix access to sf_dev_table on allocation failure

Even when SF devices are supported, the SF device table allocation
can still fail.
In such case mlx5_sf_dev_supported still reports true, but SF device
table is invalid. This can result in NULL table access.

Hence, fix it by adding NULL table check.

Fixes: 1958fc2f ("net/mlx5: SF, Add auxiliary device driver")
Signed-off-by: Shay Drory <shayd@nvidia.com>
Reviewed-by: Parav Pandit <parav@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>