net: stmmac: remove unused PCS loopback support

net: stmmac: remove unused PCS loopback support

Nothing calls stmmac_pcs_ctrl_ane() with the "loopback" argument set to
anything except zero, so this serves no useful purpose. Remove the
argument to reduce the code complexity.

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/E1v9P6I-0000000Aola-3Sih@rmk-PC.armlinux.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>