usb: gadget: mass_storage: added sysfs entry for cdrom to LUNs [FrozenCow]

This patch adds a cdrom sysfs entry for each mass_storage LUN, just like ro sysfs entry. This allows switching between USB and CD-ROM emulation without reinserting the module or recompiling the kernel.

Change-Id: Idf83c74815b1ad370428ab9d3e5503d5f7bcd3b6