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 UMS. An application that uses this system is DriveDroid (https://play.google.com/store/apps/details?id=com.softwarebakery.drivedroid).

It might be that another patch is also required, but for most modern kernels this isn't needed.