staging: rtl8712: Use fallthrough pseudo-keyword

Replace the existing /* fall through */ comments and its variants 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>
Link: https://lore.kernel.org/r/20200727185838.GA29595@embeddedorSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/rtl8712/usb_ops_linux.c

@@ -225,7 +225,7 @@ static void r8712_usb_read_port_complete(struct urb *purb)
 				padapter->driver_stopped = true;
 				break;
 			}
-			/* Fall through. */
+			fallthrough;
 		case -EPROTO:
 			r8712_read_port(padapter, precvpriv->ff_hwaddr, 0,
 				  (unsigned char *)precvbuf);