realtek: phy: optimize RTL8214FC register usage during media set

realtek: phy: optimize RTL8214FC register usage during media set

There are some shortcomings in rtl8214fc_media_set()

- It always uses the mdio raw page (4095) of RTL838x for writes.
  That is wrong when the phy is attached to an RTL839x (raw = 8191)
- It uses the internal write only extended page companion (29)
- The extended page content is not preserved

Fix the three issues.

Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>
Link: https://github.com/openwrt/openwrt/pull/21582
Signed-off-by: Robert Marko <robimarko@gmail.com>