ramips: pinctrl: return proper error if pinctrl0 is empty

Children of the pinctrl0 node are optional. Return EINVAL (=missing)
instead of 0. Fixes a hang if the pinctrl0 has no children.

Signed-off-by: Mathias Kresin <dev@kresin.me>
Acked-by: John Crispin <john@phrozen.org>

target/linux/ramips/patches-4.9/0025-pinctrl-ralink-add-pinctrl-driver.patch

@@ -193,7 +193,7 @@ Signed-off-by: John Crispin <blogic@openwrt.org>
 +	}
 +
 +	if (!max_maps)
-+		return max_maps;
++		return -EINVAL;
 +
 +	*map = kzalloc(max_maps * sizeof(struct pinctrl_map), GFP_KERNEL);
 +	if (!*map)