ixgbe: Restore AIM support

AIM (adaptive interrupt moderation) was part of BSD11 driver. Upon IFLIB
migration, AIM feature got lost. Re-introducing AIM back into IFLIB
based IXGBE driver.

One caveat is that in BSD11 driver, a queue comprises both Rx and Tx
ring. Starting from BSD12, Rx and Tx have their own queues and rings.
Also, IRQ is now only configured for Rx side. So, when AIM is
re-enabled, we should now consider only Rx stats for configuring EITR
register in contrast to BSD11 where Rx and Tx stats were considered to
manipulate EITR register.

Reviewed by: gallatin, markj
Sponsored by: NetApp, Inc.
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D27344