net: ena: fix missing lock during device destruction

acquire the rtnl_lock during device destruction to avoid
using partially destroyed device.

ena_remove() shares almost the same logic as ena_destroy_device(),
so use ena_destroy_device() and avoid duplications.

Signed-off-by: Netanel Belgazal <netanel@amazon.com>
Signed-off-by: David S. Miller <davem@davemloft.net>