USB: f_mass_storage: Added cdrom option to UMS-lun-devices

This patch allows programs to change the cdrom option at runtime
through sysfs.

Change-Id: Id2a02ab67062734793424c32c94024d6bbaccac6
Merge pull request #7 from FrozenCow/tkoonsez_cdrom_switch

USB: f_mass_storage: Added cdrom option to UMS-lun-devices