realtek: align declaration/static/extern function definitions for eth driver

realtek: align declaration/static/extern function definitions for eth driver

In 2023 upstream has tightened compiler checks with this patch
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit
/scripts/Makefile.extrawarn?h=v6.15&id=0fcb70851fbfea1776ae62f67c503fef8f0292b9

A consistent function definition is now needed. I.e.

- functions must be either declared with "static" or
- functions need an additional declaration (in the header file)

Before upgrading the Realtek target to 6.12 clean the code so that the main
6.12 PR can focus on real issues from the version bump.

Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>
Link: https://github.com/openwrt/openwrt/pull/18925
Signed-off-by: Robert Marko <robimarko@gmail.com>