net: phy: realtek: simplify C22 reg access via MDIO_MMD_VEND2

net: phy: realtek: simplify C22 reg access via MDIO_MMD_VEND2

RealTek 2.5GE PHYs have all standard Clause-22 registers mapped also
inside MDIO_MMD_VEND2 at offset 0xa400. This is used mainly in case the
PHY is connected to a Clause-45-only bus. The RTL8221B is frequently
used in copper SFP module which uses the RollBall MDIO-over-I2C
method which *only* supports Clause-45, for example.

In order to support using the PHY on Clause-45-only busses, the PHY
driver has previously been split into a C22-only and C45-only instances,
creating quite a bit of redundancy and confusion.

In preparation of reunifying the two driver instances, add support for
translating MDIO_MMD_VEND2 registers 0xa400 to 0xa43c back to Clause-22
registers 0 to 30 in case the PHY is accessed on a Clause-22 bus.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Link: https://patch.msgid.link/fd49d86bd0445b76269fd3ea456c709c2066683f.1768275364.git.daniel@makrotopia.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>