net: stmmac: Fix unset max_speed difference between DT and non-DT platforms

In commit 9cbadf094d ("net: stmmac: support max-speed device tree
property"), when DT platforms don't set "max-speed", max_speed is set to
-1; for non-DT platforms, it stays the default 0.

Prior to commit eeef2f6b9f ("net: stmmac: Start adding phylink support"),
the check for a valid max_speed setting was to check if it was greater
than zero. This commit got it right, but subsequent patches just checked
for non-zero, which is incorrect for DT platforms.

In commit 92c3807b9a ("net: stmmac: convert to phylink_get_linkmodes()")
the conversion switched completely to checking for non-zero value as a
valid value, which caused 1000base-T to stop getting advertised by
default.

Instead of trying to fix all the checks, simply leave max_speed alone if
DT property parsing fails.

Fixes: 9cbadf094d ("net: stmmac: support max-speed device tree property")
Fixes: 92c3807b9a ("net: stmmac: convert to phylink_get_linkmodes()")
Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Acked-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Link: https://lore.kernel.org/r/20220331184832.16316-1-wens@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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

@@ -431,8 +431,7 @@ stmmac_probe_config_dt(struct platform_device *pdev, u8 *mac)
 	plat->phylink_node = np;
 
 	/* Get max speed of operation from device tree */
-	if (of_property_read_u32(np, "max-speed", &plat->max_speed))
-		plat->max_speed = -1;
+	of_property_read_u32(np, "max-speed", &plat->max_speed);
 
 	plat->bus_id = of_alias_get_id(np, "ethernet");
 	if (plat->bus_id < 0)