USB: serial: 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/latest/process/deprecated.html?highlight=fallthrough#implicit-switch-case-fall-throughSigned-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Johan Hovold <johan@kernel.org>

drivers/usb/serial/cypress_m8.c

@@ -1046,7 +1046,7 @@ static void cypress_read_int_callback(struct urb *urb)
 		return;
 	case -EPIPE:
 		/* Can't call usb_clear_halt while in_interrupt */
-		/* FALLS THROUGH */
+		fallthrough;
 	default:
 		/* something ugly is going on... */
 		dev_err(dev, "%s - unexpected nonzero read status received: %d\n",
@@ -1195,7 +1195,7 @@ static void cypress_write_int_callback(struct urb *urb)
 		return;
 	case -EPIPE:
 		/* Cannot call usb_clear_halt while in_interrupt */
-		/* FALLTHROUGH */
+		fallthrough;
 	default:
 		dev_err(dev, "%s - unexpected nonzero write status received: %d\n",
 			__func__, status);

drivers/usb/serial/io_edgeport.c

@@ -1752,7 +1752,7 @@ static void process_rcvd_data(struct edgeport_serial *edge_serial,
 				edge_serial->rxState = EXPECT_HDR2;
 				break;
 			}
-			/* Fall through */
+			fallthrough;
 		case EXPECT_HDR2:
 			edge_serial->rxHeader2 = *buffer;
 			++buffer;
@@ -1804,7 +1804,7 @@ static void process_rcvd_data(struct edgeport_serial *edge_serial,
 				edge_serial->rxState = EXPECT_DATA;
 				break;
 			}
-			/* Fall through */
+			fallthrough;
 		case EXPECT_DATA: /* Expect data */
 			if (bufferLength < edge_serial->rxBytesRemaining) {
 				rxLen = bufferLength;

drivers/usb/serial/kobil_sct.c

@@ -499,7 +499,7 @@ static void kobil_set_termios(struct tty_struct *tty,
 		break;
 	default:
 		speed = 9600;
-		/* fall through */
+		fallthrough;
 	case 9600:
 		urb_val = SUSBCR_SBR_9600;
 		break;

drivers/usb/serial/upd78f0730.c

@@ -332,7 +332,7 @@ static void upd78f0730_set_termios(struct tty_struct *tty,
 		tty->termios.c_cflag &= ~CSIZE;
 		tty->termios.c_cflag |= CS8;
 		dev_warn(dev, "data size is not supported, using 8 bits\n");
-		/* fall through */
+		fallthrough;
 	case CS8:
 		request.params |= UPD78F0730_DATA_SIZE_8_BITS;
 		dev_dbg(dev, "%s - 8 data bits\n", __func__);