greybus: unlock hd mutex on free

When free_hd() is called, hd_mutex is held.  It is the
responsibility of free_hd() to drop that mutex.
Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>

drivers/staging/greybus/core.c

@@ -294,6 +294,7 @@ static void free_hd(struct kref *kref)
 	hd = container_of(kref, struct greybus_host_device, kref);
 
 	kfree(hd);
+	mutex_unlock(&hd_mutex);
 }
 
 struct greybus_host_device *greybus_create_hd(struct greybus_host_driver *driver,