net: ena: add intr_moder_rx_interval to struct ena_com_dev and use it

Add intr_moder_rx_interval to struct ena_com_dev and use it as the
location where the interrupt moderation rx interval is saved, instead
of the interrupt moderation table.

This is done as a first step before removing the old interrupt moderation
code.

Signed-off-by: Arthur Kiyanovski <akiyano@amazon.com>
Signed-off-by: David S. Miller <davem@davemloft.net>