USB: g_file_storage: don't generate automatic serial string

This patch (as1413) changes g_file_storage to avoid generating a bogus
automatic serial-number string descriptor.  If the user doesn't provide
a valid serial number via a module parameter then a warning is logged
and the gadget won't have any serial string descriptor at all.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Acked-by: David Brownell <david-b@pacbell.net>
CC: Michal Nazarewicz <m.nazarewicz@samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>