greybus: nullify dangling pointers

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>
diff --git a/drivers/staging/greybus/debugfs.c b/drivers/staging/greybus/debugfs.c
index ef292f4..4755a36 100644
--- a/drivers/staging/greybus/debugfs.c
+++ b/drivers/staging/greybus/debugfs.c
@@ -26,4 +26,5 @@
 void gb_debugfs_cleanup(void)
 {
 	debugfs_remove_recursive(gb_debug_root);
+	gb_debug_root = NULL;
 }