net/mlx5: Fix possible deadlock on rule deletion

Add missing call to up_write_ref_node() which releases the semaphore
in case the FTE doesn't have destinations, such in drop rule case.

Fixes: 465e7baa

 ("net/mlx5: Fix deletion of duplicate rules")
Signed-off-by: Maor Gottlieb <maorg@nvidia.com>
Reviewed-by: Mark Bloch <mbloch@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>