USB: serial: spcp8x5: fix modem-status handling

commit 5ed8d410 upstream.

Make sure to detect short control transfers and return zero on success
when retrieving the modem status.

This fixes the TIOCMGET implementation which since e1ed212d ("USB:
spcp8x5: add proper modem-status support") has returned TIOCM_LE on
successful retrieval, and avoids leaking bits from the stack on short
transfers.

This also fixes the carrier-detect implementation which since the above
mentioned commit unconditionally has returned true.

Fixes: e1ed212d ("USB: spcp8x5: add proper modem-status support")
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/serial/spcp8x5.c

@@ -232,11 +232,17 @@ static int spcp8x5_get_msr(struct usb_serial_port *port, u8 *status)
 	ret = usb_control_msg(dev, usb_rcvctrlpipe(dev, 0),
 			      GET_UART_STATUS, GET_UART_STATUS_TYPE,
 			      0, GET_UART_STATUS_MSR, buf, 1, 100);
-	if (ret < 0)
+	if (ret < 1) {
 		dev_err(&port->dev, "failed to get modem status: %d\n", ret);
+		if (ret >= 0)
+			ret = -EIO;
+		goto out;
+	}
 
 	dev_dbg(&port->dev, "0xc0:0x22:0:6  %d - 0x02%x\n", ret, *buf);
 	*status = *buf;
+	ret = 0;
+out:
 	kfree(buf);
 
 	return ret;