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

Author: FrozenCow <frozencow@gmail.com>

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: I9997b76c49f47bbbaf4216f6927ee43a9a436db1