USB: serial: metro-usb: stop I/O after failed open

USB: serial: metro-usb: stop I/O after failed open

Make sure to kill the interrupt-in URB after a failed open request.
Apart from saving power (and avoiding stale input after a later
successful open), this also prevents a NULL-deref in the completion
handler if the port is manually unbound.

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Fixes: 704577861d5e ("USB: serial: metro-usb: get data from device in Uni-Directional mode.")
Cc: stable <stable@vger.kernel.org>	# 3.5
Signed-off-by: Johan Hovold <johan@kernel.org>