USB: deprecate g_file_storage

This patch (as1471) deprecates the File-backed Storage Driver and
schedules its replacement for the 3.8 kernel release (about two years
from now).  Users are advised to switch to the Mass Storage Gadget
instead.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>