phy: lynx-28g: distinguish between 10GBASE-R and USXGMII

phy: lynx-28g: distinguish between 10GBASE-R and USXGMII

The driver does not handle well protocol switching to or from USXGMII,
because it conflates it with 10GBase-R.

In the expected USXGMII use case, that isn't a problem, because SerDes
protocol switching performed by the lynx-28g driver is not necessary,
because USXGMII natively supports multiple speeds, as opposed to SFP
modules using 1000Base-X or 10GBase-R which require switching between
the 2.

That being said, let's be explicit, and in case someone requests a
protocol change which involves USXGMII, let's do the right thing.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://patch.msgid.link/20251125114847.804961-13-vladimir.oltean@nxp.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>