net: ena: fix default tx interrupt moderation interval

Current default non-adaptive tx interrupt moderation interval is 196 us.
This value is too high and might cause the tx queue to fill up.

In this commit we set the default non-adaptive tx interrupt moderation
interval to 64 us in order to:
1. Reduce the probability of the queue filling-up (when compared to the
   current default value of 196 us).
2. Reduce unnecessary tx interrupt overhead (which happens if we set the
   default tx interval to 0).
   We determined experimentally that 64 us is an optimal value that
   reduces interrupt rate by more than 20% without affecting performance.

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>