mtd: maps: gpio-addr-flash: Fix ioremapped size

We should only iomap the area of the chip that is memory mapped.
Otherwise we could be mapping devices beyond the memory space or that
belong to other devices.

Signed-off-by: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>
Fixes: ebd71e3a

 ("mtd: maps: gpio-addr-flash: fix warnings and make more portable")
Cc: <stable@vger.kernel.org>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>