net: stmmac: dwmac-socfpga: provide select_pcs() implementation

Provide a .select_pcs() implementation which returns the phylink PCS
that was created in the .pcs_init() method.

Reviewed-by: Romain Gantois <romain.gantois@bootlin.com>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://lore.kernel.org/r/E1sHhob-00FetN-Vp@rmk-PC.armlinux.org.uk


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