mtd: maps: rbtx4939-flash: fix compile error

We got the syntax wrong here. Compile tested this time!

Error:

   drivers/mtd/maps/rbtx4939-flash.c: In function 'rbtx4939_flash_probe':
>> drivers/mtd/maps/rbtx4939-flash.c:99:11: error: request for member 'dev' in something not a structure or union
     info->mtd.dev.parent = &dev->dev;
              ^

Fixes: 9aa7e502 ("mtd: maps: rbtx4939-flash: show parent device in sysfs")
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Cc: Frans Klaver <fransklaver@gmail.com>