realtek: pcs: rtl931x: read chip specifics in early init

realtek: pcs: rtl931x: read chip specifics in early init

The SerDes setup for RTL931x relies on chip specifics in some cases,
Determining both usually requires some register operations. But we can
avoid to do this every time again and again since the information is
static anyway. Thus, move this to initialization for RTL93xx, only read
once and store it in the global control structure. Though not used for
RTL930x, it has the same registers and information.

While at it, give referenced defines a proper prefix.

Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/21858
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>