realtek: rtl83xx-phy: abstract and document PHY features

realtek: rtl83xx-phy: abstract and document PHY features

Replace magic values with more self-descriptive code now that I start
to understand more about the design of the PHY (and MDIO controller).

Remove one line before reading RTL8214FC internal PHY id which turned
out to be a no-op and can hence safely be removed (confirmed by
INAGAKI Hiroshi[1])

[1]: https://github.com/openwrt/openwrt/commit/df8e6be59a1fbce3f8c6878fe7440a129b1245d6#r66890713

Signed-off-by: Daniel Golle <daniel@makrotopia.org>