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