net: stmmac: do not require snps,ps-speed for SGMII

SGMII mode does not require port-speed to be specified; this only
switches SGMII to use the MAC configuration register speed settings
and the actual value is irrelevant when the link comes up.

As it seems the intention was to support "reverse SGMII" with this
setting, but the code didn't actually configure that due to a typo,
the warning and bad DT binding documentation has led people to
specify snps,ps-speed in their DT files inappropriately.

If mac_port_sel_speed is zero, then don't complain that the speed
is invalid, as this means we're using "normal" SGMII.

This does _not_ obsolete snps,ps-speed, nor does it change the
behaviour of that property, with the exception of not making people
mistakenly believe that they need to specify this option to use
normal SGMII. There is no need to modify the binding.

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/E1v9P6n-0000000Aom9-2LuZ@rmk-PC.armlinux.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/stmicro/stmmac/stmmac_main.c

@@ -1101,6 +1101,8 @@ static void stmmac_check_pcs_mode(struct stmmac_priv *priv)
 
 		default:
 			dev_warn(priv->device, "invalid port speed\n");
+			fallthrough;
+		case 0:
 			priv->hw->reverse_sgmii_enable = false;
 			break;
 		}