net/tap: fix memory leak when unregister intr handler

The return check of function tap_lsc_intr_handle_set() is wrong, it should
be 0 or a positive number if success. So the intr_handle->intr_vec was not
been freed when tap_lsc_intr_handle_set() returned a positive number.

Fixes: 4870a8cdd968 ("net/tap: support Rx interrupt")
Cc: stable@dpdk.org
Signed-off-by: Yunjian Wang <wangyunjian@huawei.com>
Acked-by: Ferruh Yigit <ferruh.yigit@intel.com>