thunderbolt: Make it possible to allocate one directional DMA tunnel

With DMA tunnels it is possible that the service using it does not
require bi-directional paths so make RX and TX optional (but of course
one of them needs to be set).

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Acked-by: Yehezkel Bernat <YehezkelShB@gmail.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>