usb: 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>
Link: https://lore.kernel.org/r/20200707195607.GA4198@embeddedor
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/class/usbtmc.c

@@ -2282,7 +2282,7 @@ static void usbtmc_interrupt(struct urb *urb)
 	case -EOVERFLOW:
 		dev_err(dev, "overflow with length %d, actual length is %d\n",
 			data->iin_wMaxPacketSize, urb->actual_length);
-		/* fall through */
+		fallthrough;
 	case -ECONNRESET:
 	case -ENOENT:
 	case -ESHUTDOWN: