mtd: partitions: redboot: fix style issues

This patch fixes easy checkpatch issues.

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/20210520114851.1274609-2-clabbe@baylibre.com