intel_th: Fix device removal logic

Commit a753bfcf ("intel_th: Make the switch allocate its subdevices")
brings in new subdevice addition/removal logic that's broken for "host
mode": the SWITCH device has no children to begin with, which is not
handled in the code. This results in a null dereference bug later down
the path.

This patch fixes the subdevice removal code to handle host mode correctly.
Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Fixes: a753bfcf ("intel_th: Make the switch allocate its subdevices")
CC: stable@vger.kernel.org # v4.14+
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/hwtracing/intel_th/core.c

@@ -139,7 +139,8 @@ static int intel_th_remove(struct device *dev)
 			th->thdev[i] = NULL;
 		}
 
-		th->num_thdevs = lowest;
+		if (lowest >= 0)
+			th->num_thdevs = lowest;
 	}
 
 	if (thdrv->attr_group)