net: stmmac: move reverse-"pcs" mode setup to stmmac_check_pcs_mode()

net: stmmac: move reverse-"pcs" mode setup to stmmac_check_pcs_mode()

The broken reverse-mode, selected by snps,ps-speed, is configured when
the platform provides a valid port speed and a PCS is being used.

Both these remain constant after the driver has probed, so the software
state doesn't need to be re-initialised each time stmmac_hw_setup() is
called (which is called at open and resume time.)

Move the software setup of reverse-mode to stmmac_check_pcs_mode()
which is called from the driver probe function.

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/E1v9P6Y-0000000Aolr-0vLH@rmk-PC.armlinux.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>