thunderbolt: Fix kernel-doc for tb_tunnel_alloc_dp()

In case of no bandwidth available for DP tunnel, the function get the arguments
@max_up and @max_down set to zero. Fix the kernel-doc to describe more
accurately the purpose of the function.

No functional changes.

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