realtek: phy: enhance RTL8218B initialization

realtek: phy: enhance RTL8218B initialization

The RTL8214FC and the RTL8218B share the same register set and
need to be initialized quite similar. In the future the RTl8214FC
will get its own setup sequence. To keep the codebase small the
common parts (serdes & copper) will be covered by the existing
RTL8218B configuration.

Enhance the RTL8218B setup with this separate commit to ensure
that nothing breaks.

Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>
Link: https://github.com/openwrt/openwrt/pull/21435
Signed-off-by: Stijn Tintel <stijn@linux-ipv6.be>