realtek: use more specific APIs for ethtool stats where possible

realtek: use more specific APIs for ethtool stats where possible

The kernel offers several alternatives to get_ethtool_stats which allow
to report some stats in a more structured way. Use them where possible.

Ideally, we should follow RFC 3635 to translate the hardware counters to
the supported frame and octet counters. However, this is not feasible,
as some of the counters are 32-bit only (so it would produce incorrect
results as soon as one of them overflows).

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>