ionic: fix up dim accounting for tx and rx

We need to count the correct Tx and/or Rx packets for dynamic
interrupt moderation, depending on which we're processing on
the queue interrupt.

Fixes: 04a83459

 ("ionic: dynamic interrupt moderation")
Signed-off-by: Shannon Nelson <snelson@pensando.io>
Signed-off-by: David S. Miller <davem@davemloft.net>