kernel: net: phy: realtek: improve RealTek 2.5G PHY driver

 * use interface mode switching only when operating in C45 mode
   Linux prevents switching the interface mode when using C22 MDIO,
   hence use rate-adapter mode in case the PHY controlled via C22.

 * use phy_read_paged where appropriate

 * use existing generic inline functions to handle 10GbE advertisements
   instead of redundantly defining register macros in realtek.c which
   are not actually vendor-specific.

 * make sure 10GbE advertisement is valid, preventing false-positive
   warning "Downshift occurred from negotiated speed 2.5Gbps to actual
   speed 1Gbps, check cabling!" with some link-partners using 1G mode.

 * Support Link Down Power Saving Mode (ALDPS)

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