thunderbolt: Fix uninitialized variable in tb_tunnel_alloc_usb3()

Currently in case of no bandwidth available for USB3 tunnel, we are left
with uninitialized variable that can lead to huge negative allocated
bandwidth.

Fix this by initializing the variable to zero. While there, fix the
kernel-doc to describe more accurately the purpose of the function
tb_tunnel_alloc_usb3().
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Closes: https://lore.kernel.org/linux-usb/6289898b-cd63-4fb8-906a-1b6977321af9@moroto.mountain/
Fixes: 25d905d2 ("thunderbolt: Allow USB3 bandwidth to be lower than maximum supported")
Signed-off-by: Gil Fine <gil.fine@linux.intel.com>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>

drivers/thunderbolt/tunnel.c

@@ -2048,10 +2048,10 @@ struct tb_tunnel *tb_tunnel_discover_usb3(struct tb *tb, struct tb_port *down,
  * @tb: Pointer to the domain structure
  * @up: USB3 upstream adapter port
  * @down: USB3 downstream adapter port
- * @max_up: Maximum available upstream bandwidth for the USB3 tunnel (%0
- *	    if not limited).
- * @max_down: Maximum available downstream bandwidth for the USB3 tunnel
- *	      (%0 if not limited).
+ * @max_up: Maximum available upstream bandwidth for the USB3 tunnel.
+ *	    %0 if no available bandwidth.
+ * @max_down: Maximum available downstream bandwidth for the USB3 tunnel.
+ *	      %0 if no available bandwidth.
  *
  * Allocate an USB3 tunnel. The ports must be of type @TB_TYPE_USB3_UP and
  * @TB_TYPE_USB3_DOWN.
@@ -2064,7 +2064,7 @@ struct tb_tunnel *tb_tunnel_alloc_usb3(struct tb *tb, struct tb_port *up,
 {
 	struct tb_tunnel *tunnel;
 	struct tb_path *path;
-	int max_rate;
+	int max_rate = 0;
 
 	if (!tb_route(down->sw) && (max_up > 0 || max_down > 0)) {
 		/*