USB: serial: mos7720: fix control-message error handling

Make sure to log an error on short transfers when reading a device
register.

Also clear the provided buffer (which if often an uninitialised
automatic variable) on errors as the driver currently does not bother to
check for errors.
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>

drivers/usb/serial/mos7720.c

@@ -234,11 +234,16 @@ static int read_mos_reg(struct usb_serial *serial, unsigned int serial_portnum,
 
 	status = usb_control_msg(usbdev, pipe, request, requesttype, value,
 				     index, buf, 1, MOS_WDR_TIMEOUT);
-	if (status == 1)
+	if (status == 1) {
 		*data = *buf;
-	else if (status < 0)
+	} else {
 		dev_err(&usbdev->dev,
 			"mos7720: usb_control_msg() failed: %d\n", status);
+		if (status >= 0)
+			status = -EIO;
+		*data = 0;
+	}
+
 	kfree(buf);
 
 	return status;