realtek: implement polling for hardware counters

realtek: implement polling for hardware counters

Maintain 64 bit counters by polling the hardware counters and adding up
the differences. Polling needs to happen just often enough to catch
every single overflow.

As we now have non-overflowing counters now, we can safely calculate
composite counters without getting weird results on overflow. Use this
to follow RFC 3635 more accurately by mapping the hardware counters to
the proper counters, while taking into account hardware quirks as best
as possible.

Signed-off-by: Jan Hoffmann <jan@3e8.eu>
Link: https://github.com/openwrt/openwrt/pull/18415
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>