USB: f_mass_storage: 2048 block size for cdrom devices

Bios will recognize the cdrom device like most other cdrom
devices. This allows people to boot ISOs from their phone.

Original by Andreas Kemnade:
http://www.spinics.net/lists/linux-usb/msg25178.html

Change-Id: Id8fba96e1683f6daa6330b1e59f4a4ca4fe5c271