thunderbolt: Fix calculation of consumed USB3 bandwidth on a path

Currently, when setup a new USB3 tunnel that is starting from downstream USB3
adapter of first depth router (or deeper), to upstream USB3 adapter of a second
depth router (or deeper), we calculate consumed bandwidth. For this calculation
we take into account first USB3 tunnel consumed bandwidth while we shouldn't,
because we just recalculating the first USB3 tunnel allocated bandwidth.

Fix that, so that more bandwidth is available for the new USB3 tunnel being
setup.

While there, fix the kernel-doc to decribe more accurately the purpose of the
function.

Signed-off-by: Gil Fine <gil.fine@linux.intel.com>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>