isp1760-hcd: fix possible memory leak if urb could not be enqueued

After packetize_urb was called, we could still run into an error path
and will not hand over the prepared qtd to the qtd_list. Make sure to
free the prepared qtd in that case to avoid memory leaks.

Signed-off-by: Michael Grzeschik <m.grzeschik@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>