net: stmmac: remove RGMII "pcs" mode

net: stmmac: remove RGMII "pcs" mode

Remove the RGMII "pcs" code in stmmac_check_pcs_mode() due to:

1) This should never have been conditional on a PCS being present, as
   when a core is synthesised using only RGMII, the PCS won't be present
   and priv->dma_cap.pcs will be false. Only multi-interface cores which
   have a PCS present would have detected RGMII.

2) STMMAC_PCS_RGMII has no effect since the broken netif_carrier and
   ethtool code was removed.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Tested-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Tested-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Link: https://patch.msgid.link/E1v9P6T-0000000Aoll-0Ify@rmk-PC.armlinux.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>