USB: Add a serial number parameter to g_file_storage module

This patch add a serial number parameter to the g_file_storage
module. There's validity checks against the string passed to comply
with the specs.

Signed-off-by: Yann Cantin <yann.cantin@laposte.net>
Cc: Michał Nazarewicz <m.nazarewicz@samsung.com>
Cc: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>