While accessing a unusual usb storage (ums-alauda, ums-cypress, ...),
the module reference count is not incremented. Because these drivers
allocate scsi hosts with usb_stor_host_template defined in usb-storage
module. So these drivers always can be unloaded.
This fixes it by preparing scsi host template which is initialized
at module_init() for each ums-* driver. In order to minimize the
difference in ums-* drivers, introduce module_usb_stor_driver() helper
macro which is same as module_usb_driver() except that it also
initializes scsi host template.