mtd: Fix C++ comment in include/linux/mtd/mtd.h

C++ comments look wrong in kernel tree. Fix one.

Signed-off-by: Pavel Machek <pavel@ucw.cz>
Acked-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Richard Weinberger <richard@nod.at>