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.

Signed-off-by: Michael Bestas <mikeioannina@gmail.com>

Change-Id: I99e60379ccfcca3b46192e9a7403704e18abbcbd