USB: android: check for platform data presence before accessing it

commit c9cb205f (USB: android: Add a new LUN as CD-ROM in mass-storage)
access platform data without checking for NULL.  This causes a kernel
panic on targets which don't have platform data.

Change-Id: I4542b0dce81604706f90b7f6e851ed47c04a941c
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>