watchdog: Fix fall-through warnings for Clang

In preparation to enable -Wimplicit-fallthrough for Clang, fix a warning
by explicitly adding a fallthrough pseudo-keyword instead of letting the
code fall through to the next case.

Link: KSPP#115
Signed-off-by: Gustavo A. R. Silva <[email protected]>

Author: GustavoARSilva

drivers/watchdog/machzwd.c

@@ -174,6 +174,7 @@ static inline void zf_set_timer(unsigned short new, unsigned char n)
 		fallthrough;
 	case WD2:
 		zf_writeb(COUNTER_2, new > 0xff ? 0xff : new);
+		fallthrough;
 	default:
 		return;
 	}