net: stmmac: dwmac-intel: provide a select_pcs() implementation

Move the code returning the XPCS into dwmac-intel, which is the only
user of XPCS. Fill in the select_pcs() implementation only when we are
going to setup the XPCS, thus when it should be present.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://lore.kernel.org/r/E1sHhoR-00FetB-CP@rmk-PC.armlinux.org.uk


Signed-off-by: Jakub Kicinski <kuba@kernel.org>