mtd: maps: sa1100-flash: potential NULL dereference

We check for NULL but then dereference "info->mtd" on the next line.

Fixes: 72169755 ('mtd: maps: sa1100-flash: show parent device in sysfs')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>