ubi: fastmap: fix spelling mistake: "invalidiate" -> "invalidate"

Trivial fix to spelling mistake in ubi_err error message

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Richard Weinberger <richard@nod.at>