net/ena: fix queues number calculation

The ENA driver tries to open a queue per vCPU.
To determine how many vCPUs the instance have it uses num_possible_cpus()
while it should have use num_online_cpus() instead.

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