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

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: I30a0ffc7f6e9a7f12d95ca1ab07949868aba440c
Merge pull request #9 from FrozenCow/frozencow_munjeni_xperiago_cm-11_gadget_cdrom

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