treewide: Use fallthrough pseudo-keyword

Replace the existing /* fall through */ comments and its variants with
the new pseudo-keyword macro fallthrough[1]. Also, remove unnecessary
fall-through markings when it is the case.

[1] https://www.kernel.org/doc/html/v5.7/process/deprecated.html?highlight=fallthrough#implicit-switch-case-fall-through

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>

drivers/mux/adgs1408.c

@@ -93,7 +93,7 @@ static int adgs1408_probe(struct spi_device *spi)
 			mux->idle_state = idle_state;
 			break;
 		}
-		/* fall through */
+		fallthrough;
 	default:
 		dev_err(dev, "invalid idle-state %d\n", idle_state);
 		return -EINVAL;