net: stmmac: Make stmmac_xpcs_setup() generic to all PCS devices

A pcs_init() callback will be introduced to stmmac in a future patch. This
new function will be called during the hardware initialization phase.
Instead of separately initializing XPCS and PCS components, let's group all
PCS-related hardware initialization logic in the current
stmmac_xpcs_setup() function.

Rename stmmac_xpcs_setup() to stmmac_pcs_setup() and move the conditional
call to stmmac_xpcs_setup() inside the function itself.

Signed-off-by: Serge Semin <fancer.lancer@gmail.com>
Co-developed-by: Romain Gantois <romain.gantois@bootlin.com>
Signed-off-by: Romain Gantois <romain.gantois@bootlin.com>
Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Hariprasad Kelam <hkelam@marvell.com>
Link: https://lore.kernel.org/r/20240513-rzn1-gmac1-v7-3-6acf58b5440d@bootlin.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>