net: ena: Set default value for RX interrupt moderation

RX ring can be NULL in XDP use cases where only TX queues
are configured. In this scenario, the RX interrupt moderation
value sent to the device remains in its default value of 0.

In this change, setting the default value of the RX interrupt
moderation to be the same as of the TX.

Fixes: 548c4940

 ("net: ena: Implement XDP_TX action")
Signed-off-by: David Arinzon <darinzon@amazon.com>
Signed-off-by: David S. Miller <davem@davemloft.net>