mtd: partitions: redboot: seek fis-index-block in the right node

fis-index-block is seeked in the master node and not in the partitions node.
For following binding and current usage, the driver need to check the
partitions subnode.

Fixes: c0e118c8 ("mtd: partitions: Add OF support to RedBoot partitions")
Signed-off-by: Corentin Labbe <clabbe@baylibre.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20210520114851.1274609-1-clabbe@baylibre.com