video: fbdev: fsl-diu-fb: fix spelling mistake "palette"

trivial fix to spelling mistakes of "palette"

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Timur Tabi <timur@tabi.org>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>