mtd: maps: add missing iounmap() in error path

This patch was triggered by the following Coccinelle error:

./drivers/mtd/maps/sc520cdp.c:246:3-9: \
	ERROR: missing iounmap; ioremap on line 242 \
	and execution via conditional on line 244

Since do_map_probe() is also invoked in this loop, it is also necessary to
map_destroy() any initialised struct mtd_info.

Signed-off-by: Luis Henriques <henrix@camandro.org>
Reviewed-by: Marek Vasut <marek.vasut@gmail.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>