net/tap: fix cleanup on allocation failure

This patch complements the partial cleanup done inside
eth_dev_tap_create when the routine failed.
Such a failure left a non-functional device attached to the system.

Fixes: 050fe6e9ff97 ("drivers/net: use ethdev allocation helper for vdev")
Cc: stable@dpdk.org
Signed-off-by: Moti Haimovsky <motih@mellanox.com>
Acked-by: Pascal Mazon <pascal.mazon@6wind.com>