iio: adc: twl6030-gpadc: Use the defined variable to clean code

Use the defined variable "dev" to make the code cleaner.

Signed-off-by: Tang Bin <tangbin@cmss.chinamobile.com>
Link: https://lore.kernel.org/r/20210823095921.16828-1-tangbin@cmss.chinamobile.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>