use the kernel's way of suppressing fallthrough

See include/linux/compiler_attributes.h:

/*
 * Add the pseudo keyword 'fallthrough' so case statement blocks
 * must end with any of these keywords:
 *   break;
 *   fallthrough;
 *   goto <label>;
 *   return [expression];
 *
 *  gcc: https://gcc.gnu.org/onlinedocs/gcc/Statement-Attributes.html#Statement-Attributes
 */
-if __has_attribute(__fallthrough__)
- define fallthrough                    __attribute__((__fallthrough__))
-else
- define fallthrough                    do {} while (0)  /* fallthrough */
-endif

ac108.c

@@ -865,7 +865,7 @@ static int ac108_set_fmt(struct snd_soc_dai *dai, unsigned int fmt) {
 			/* TODO: Both cpu_dai and codec_dai(AC108) be set as slave in DTS */
 			dev_dbg(dai->dev, "used as slave when AC101 is master\n");
 		}
-		/* fall through */
+		fallthrough;
 	case SND_SOC_DAIFMT_CBS_CFS:    /*AC108 Slave*/
 		dev_dbg(dai->dev, "AC108 set to work as Slave\n");
 		/**

wm8960.c

@@ -752,8 +752,8 @@ static int wm8960_hw_params(struct snd_pcm_substream *substream,
 		if ((iface & 0x3) != 0) {
 			iface |= 0x000c;
 			break;
-		};
-		/* fall through */
+		}
+		fallthrough;
 	default:
 		dev_err(codec->dev, "unsupported width %d\n",
 			params_width(params));