net: ena: Add first_interrupt field to napi struct

The first_interrupt field is accessed in ena_intr_msix_io() upon
receiving an interrupt.The rx_ring and tx_ring fields of napi can
be NULL when receiving interrupt for xdp queues. This patch fixes
the issue by moving the field to the ena_napi struct.

Signed-off-by: Sameeh Jubran <sameehj@amazon.com>
Signed-off-by: David S. Miller <davem@davemloft.net>