net/mlx5: Fix 'DON'T_TRAP' functionality

The flow counters binding support commit introduced a code change where
none NULL 'rule_dest' is always passed to mlx5_add_flow_rules, this breaks
'DON'T_TRAP' rules insertion.

The fix uses the equivalent 'dest_num' value instead of dest pointer
at the failed check.

fixes: 3b3233fb ('IB/mlx5: Add flow counters binding support')
Signed-off-by: Raed Salem <raeds@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>