net: ena: Fix error handling in ena_init()

The ena_init() won't destroy workqueue created by
create_singlethread_workqueue() when pci_register_driver() failed.
Call destroy_workqueue() when pci_register_driver() failed to prevent the
resource leak.

Fixes: 1738cd3e

 ("net: ena: Add a driver for Amazon Elastic Network Adapters (ENA)")
Signed-off-by: Yuan Can <yuancan@huawei.com>
Acked-by: Shay Agroskin <shayagr@amazon.com>
Link: https://lore.kernel.org/r/20221114025659.124726-1-yuancan@huawei.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>