net/af_xdp: fix remove path

net/af_xdp: fix remove path

When users call rte_eth_dev_close() and rte_dev_remove(), the af_xdp
pmd return -1 (EPERM) due to eth_dev == NULL.

Since the af_xdp pmd driver advertises RTE_ETH_DEV_CLOSE_REMOVE, all
the resources are freed on rte_eth_dev_close().  rte_dev_remove() tries
to detach device and subsequently calls rte_pmd_af_xdp_remove() that
tries to free already freed resources and fails.
Fix it by return success.

Fixes: f1debd77efaf ("net/af_xdp: introduce AF_XDP PMD")
Cc: stable@dpdk.org
Reported-at: https://patchwork.ozlabs.org/patch/1106528/
Suggested-by: Ilya Maximets <i.maximets@samsung.com>
Signed-off-by: William Tu <u9012063@gmail.com>
Acked-by: Xiaolong Ye <xiaolong.ye@intel.com>