mtd: parsers: ofpart: Fix refcount leak in bcm4908_partitions_fw_offset

of_find_node_by_path() returns a node pointer with refcount incremented,
we should use of_node_put() on it when not need anymore.
Add missing of_node_put() to avoid refcount leak.

Fixes: bb17230c ("mtd: parsers: ofpart: support BCM4908 fixed partitions")
Signed-off-by: Miaoqian Lin <linmq006@gmail.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20220605070726.5979-1-linmq006@gmail.com