thunderbolt: Allow passing NULL to tb_ctl_free()

Following the usual pattern used in many places, we allow passing NULL
pointer to tb_ctl_free(). Then the user can call the function regardless
if it has allocated control channel or not making the code bit simpler.

Suggested-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Andreas Noever <andreas.noever@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/thunderbolt/ctl.c

@@ -520,6 +520,10 @@ err:
 void tb_ctl_free(struct tb_ctl *ctl)
 {
 	int i;
+
+	if (!ctl)
+		return;
+
 	if (ctl->rx)
 		ring_free(ctl->rx);
 	if (ctl->tx)