iio: potentiometer: mcp4018: use the correct MODULE_LICENSE

The file is GPL v2 only.
Signed-off-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/potentiometer/mcp4018.c

@@ -187,4 +187,4 @@ module_i2c_driver(mcp4018_driver);
 
 MODULE_AUTHOR("Peter Rosin <peda@axentia.se>");
 MODULE_DESCRIPTION("MCP4018 digital potentiometer");
-MODULE_LICENSE("GPL");
+MODULE_LICENSE("GPL v2");