mtd: fix typos in ooblayout comment blocks

- "This functions return ..." -> "This function returns ..."
 - "I you want ..." -> "If you want ..."

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>