Rework counting of hardware statistics in ENA driver

Do not read all statistics from the device, instead count them in the
driver except from RX drops - they are received directly from the NIC
in the AENQ descriptor.

Submitted by: Michal Krawczyk <mk@semihalf.com>
Reviewed by: imp
Obtained from: Semihalf
Sponsored by: Amazon.com, Inc.
Differential Revision: https://reviews.freebsd.org/D12852