ionic: remove intr coalesce update from napi

Move the interrupt coalesce value update out of the napi
thread and into the dim_work thread and set it only when it
has actually changed.

Fixes: 04a83459 ("ionic: dynamic interrupt moderation")
Signed-off-by: Shannon Nelson <snelson@pensando.io>
Signed-off-by: David S. Miller <davem@davemloft.net>