USB: serial: ir-usb: fix IrLAP framing

commit 38c0d5bd upstream.

Commit f4a4cbb2 ("USB: ir-usb: reimplement using generic framework")
switched to using the generic write implementation which may combine
multiple write requests into larger transfers. This can break the IrLAP
protocol where end-of-frame is determined using the USB short packet
mechanism, for example, if multiple frames are sent in rapid succession.

Fixes: f4a4cbb2 ("USB: ir-usb: reimplement using generic framework")
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>