realtek: fix ethtool stats for RTL839x and RTL930x

realtek: fix ethtool stats for RTL839x and RTL930x

The MIB registers contain different stats depending on the SoC, and for
RTL930x some stats are in an additional register.

Create separate MIB descs for each SoC to implement this. Also make
reading 64-bit counters more robust, by protecting against an overflow
of the lower 32 bits during the read.

RTL931x remains unsupported, because it uses a table and thus requires
a separate implementation.

While we are at it, rename structs/functions to use the rtldsa prefix.

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>