usbnet: Use fallthrough pseudo-keyword

Replace // FALLTHROUGH comment with the new pseudo-keyword macro
fallthrough[1].

[1] https://www.kernel.org/doc/html/v5.7/process/deprecated.html?highlight=fallthrough#implicit-switch-case-fall-throughSigned-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/usb/usbnet.c

@@ -597,7 +597,7 @@ static void rx_complete (struct urb *urb)
 	case -EPIPE:
 		dev->net->stats.rx_errors++;
 		usbnet_defer_kevent (dev, EVENT_RX_HALT);
-		// FALLTHROUGH
+		fallthrough;
 
 	/* software-driven interface shutdown */
 	case -ECONNRESET:		/* async unlink */