USB: fix wrong order of events in usb serial suspension

if a subdriver has an additional suspend method, it must be called
first to allow the subdriver to return -EBUSY, because the second
half cannot be easily undone.

Signed-off-by: Oliver Neukum <oliver@neukum.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>