usb: gadget: android: Allow multi-instance config of mass_storage

Android composite driver is dealing with mass_storage assuming
only one instance of it will be used. Driver allocates function
instance and gets the function once during init. Hence multiple
bind_config call result in multiple invocations of
usb_add_function causing crashes later. Also, current design
does mass-storage LUN initialization during bind_config.
This should be done only once otherwise user has to echo LUN
file name after every bind_config.

Change-Id: Id1bdae902ca790ac92692508542a3842d1cae8f3
Signed-off-by: Manu Gautam <mgautam@codeaurora.org>