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-through

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

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;