ionic: dynamic interrupt moderation

Use the dim library to manage dynamic interrupt
moderation in ionic.

v3: rebase
v2: untangled declarations in ionic_dim_work()

Signed-off-by: Shannon Nelson <snelson@pensando.io>
Acked-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>