usb: gadget: mass_storage: added sysfs entry for cdrom to LUNs

This patch should allow people to use their phone as a CD-rom device as well as a USB-disk device through USB Mass Storage. An application that uses this system is DriveDroid (https://play.google.com/store/apps/details?id=com.softwarebakery.drivedroid).

I'm not 100% sure whether I got the right branch, so please say so if I need to do the pull request for another branch.