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>

drivers/thunderbolt/tunnel.c

@@ -1435,10 +1435,10 @@ struct tb_tunnel *tb_tunnel_discover_dp(struct tb *tb, struct tb_port *in,
  * @in: DP in adapter port
  * @out: DP out adapter port
  * @link_nr: Preferred lane adapter when the link is not bonded
- * @max_up: Maximum available upstream bandwidth for the DP tunnel (%0
- *	    if not limited)
- * @max_down: Maximum available downstream bandwidth for the DP tunnel
- *	      (%0 if not limited)
+ * @max_up: Maximum available upstream bandwidth for the DP tunnel.
+ *	    %0 if no available bandwidth.
+ * @max_down: Maximum available downstream bandwidth for the DP tunnel.
+ *	      %0 if no available bandwidth.
  *
  * Allocates a tunnel between @in and @out that is capable of tunneling
  * Display Port traffic.