USB: serial: quatech2: fix control-message error handling

Make sure to detect short control-message transfers when fetching
modem and line state in open and when retrieving registers.

This specifically makes sure that an errno is returned to user space on
errors in TIOCMGET instead of a zero bitmask.

Also drop the unused getdevice function which also lacked appropriate
error handling.

Fixes: f7a33e60 ("USB: serial: add quatech2 usb to serial driver")
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>

drivers/usb/serial/quatech2.c

@@ -188,22 +188,22 @@ static inline int qt2_setdevice(struct usb_device *dev, u8 *data)
 }
 
 
-static inline int qt2_getdevice(struct usb_device *dev, u8 *data)
-{
-	return usb_control_msg(dev, usb_rcvctrlpipe(dev, 0),
-			       QT_SET_GET_DEVICE, 0xc0, 0, 0,
-			       data, 3, QT2_USB_TIMEOUT);
-}
-
 static inline int qt2_getregister(struct usb_device *dev,
 				  u8 uart,
 				  u8 reg,
 				  u8 *data)
 {
-	return usb_control_msg(dev, usb_rcvctrlpipe(dev, 0),
+	int ret;
+
+	ret = usb_control_msg(dev, usb_rcvctrlpipe(dev, 0),
 			      QT_SET_GET_REGISTER, 0xc0, reg,
 			      uart, data, sizeof(*data), QT2_USB_TIMEOUT);
+	if (ret < sizeof(*data)) {
+		if (ret >= 0)
+			ret = -EIO;
+	}
 
+	return ret;
 }
 
 static inline int qt2_setregister(struct usb_device *dev,
@@ -372,9 +372,11 @@ static int qt2_open(struct tty_struct *tty, struct usb_serial_port *port)
 				 0xc0, 0,
 				 device_port, data, 2, QT2_USB_TIMEOUT);
 
-	if (status < 0) {
+	if (status < 2) {
 		dev_err(&port->dev, "%s - open port failed %i\n", __func__,
 			status);
+		if (status >= 0)
+			status = -EIO;
 		kfree(data);
 		return status;
 	}