gpio: ath79: Fix fall-through warning for Clang

In preparation to enable -Wimplicit-fallthrough for Clang, fix a
warning by explicitly adding a fallthrough pseudo-keyword to indicate
that the code is intended to fall through to the next case.

Link: https://github.com/KSPP/linux/issues/115Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>

drivers/gpio/gpio-ath79.c

@@ -123,6 +123,7 @@ static int ath79_gpio_irq_set_type(struct irq_data *data,
 	switch (flow_type) {
 	case IRQ_TYPE_EDGE_RISING:
 		polarity |= mask;
+		fallthrough;
 	case IRQ_TYPE_EDGE_FALLING:
 	case IRQ_TYPE_EDGE_BOTH:
 		break;