mtd: parsers: ofpart: fix parsing subpartitions

ofpart was recently patched to not scan random partition nodes as
subpartitions. That change unfortunately broke scanning valid
subpartitions like:

partitions {
	compatible = "fixed-partitions";
	#address-cells = <1>;
	#size-cells = <1>;

	partition@0 {
		compatible = "fixed-partitions";
		label = "bootloader";
		reg = <0x0 0x100000>;

		partition@0 {
			label = "config";
			reg = <0x80000 0x80000>;
		};
	};
};

Fix that regression by adding 1 more code path. We actually need 3
conditional blocks to support 3 possible cases. This change also makes
code easier to understand & follow.
Reported-by: David Bauer <mail@david-bauer.net>
Fixes: 2d751203 ("mtd: parsers: ofpart: limit parsing of deprecated DT syntax
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Tested-by: Andrew Cameron <apcameron@softhome.net>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20210508173214.28365-1-zajec5@gmail.com

drivers/mtd/parsers/ofpart_core.c

@@ -57,8 +57,9 @@ static int parse_fixed_partitions(struct mtd_info *master,
 	if (!mtd_node)
 		return 0;
 
+	if (!master->parent) { /* Master */
 		ofpart_node = of_get_child_by_name(mtd_node, "partitions");
-	if (!ofpart_node && !master->parent) {
+		if (!ofpart_node) {
 			/*
 			 * We might get here even when ofpart isn't used at all (e.g.,
 			 * when using another parser), so don't be louder than
@@ -69,8 +70,9 @@ static int parse_fixed_partitions(struct mtd_info *master,
 			ofpart_node = mtd_node;
 			dedicated = false;
 		}
-	if (!ofpart_node)
-		return 0;
+	} else { /* Partition */
+		ofpart_node = mtd_node;
+	}
 
 	of_id = of_match_node(parse_ofpart_match_table, ofpart_node);
 	if (dedicated && !of_id) {