net: ena: fix NULL dereference due to untimely napi initialization

napi poll functions should be initialized before running request_irq(),
to handle a rare condition where there is a pending interrupt, causing
the ISR to fire immediately while the poll function wasn't set yet,
causing a NULL dereference.

Fixes: 1738cd3e ("net: ena: Add a driver for Amazon Elastic Network Adapters (ENA)")
Signed-off-by: Arthur Kiyanovski <akiyano@amazon.com>
Signed-off-by: David S. Miller <davem@davemloft.net>