USB: cdc-wdm: updating desc->length must be protected by spin_lock

wdm_in_callback() will also touch this field, so we cannot change it without locking

Cc: stable@vger.kernel.org
Signed-off-by: Bjørn Mork <bjorn@mork.no>
Acked-by: Oliver Neukum <oneukum@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>