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/base/firmware_loader/fallback.c

@@ -289,10 +289,10 @@ static ssize_t firmware_loading_store(struct device *dev,
 			}
 			break;
 		}
-		/* fallthrough */
+		fallthrough;
 	default:
 		dev_err(dev, "%s: unexpected value (%d)\n", __func__, loading);
-		/* fallthrough */
+		fallthrough;
 	case -1:
 		fw_load_abort(fw_sysfs);
 		break;