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/115
Signed-off-by: Gustavo A. R. Silva <[e-mail]>
Signed-off-by: Bartosz Golaszewski <[e-mail]>

drivers/gpio/gpio-ath79.c

···
 	switch (flow_type) {
 	case IRQ_TYPE_EDGE_RISING:
 		polarity |= mask;
+		fallthrough;
 	case IRQ_TYPE_EDGE_FALLING:
 	case IRQ_TYPE_EDGE_BOTH:
 		break;