thunderbolt: Fix buffer allocation of devices with no DisplayPort adapters

For the case of a device without DisplayPort adapters we calculate
incorrectly the amount of buffers. Fix the calculation for this case.
Signed-off-by: Gil Fine <gil.fine@intel.com>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>

drivers/thunderbolt/tunnel.c

@@ -102,8 +102,11 @@ static unsigned int tb_available_credits(const struct tb_port *port,
 		 * Maximum number of DP streams possible through the
 		 * lane adapter.
 		 */
-		ndp = (credits - (usb3 + pcie + spare)) /
-		      (sw->min_dp_aux_credits + sw->min_dp_main_credits);
+		if (sw->min_dp_aux_credits + sw->min_dp_main_credits)
+			ndp = (credits - (usb3 + pcie + spare)) /
+			      (sw->min_dp_aux_credits + sw->min_dp_main_credits);
+		else
+			ndp = 0;
 	} else {
 		ndp = 0;
 	}