mtd: use min_t() to refactor mtd_ooblayout_{get, set}_bytes()

I hope this will make the code a little more readable.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Marek Vasut <marek.vasut@gmail.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>