Signalling command completion from iforce_process_packet() does
not make sense, as not all transport use the same data path for
both commands and motion data form the device, that is why USB
code already has to signal command completion iforce_usb_out().

Let's move signalling completion into individual transport
modules.
Tested-by: Tim Schumacher <timschumi@gmx.de>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>