iio: adc: twl6030-gpadc: Use devm_* API family

Using devm_iio_device_alloc and devm_request_threaded_irq makes
code simpler.

Signed-off-by: Oleksandr Kozaruk <oleksandr.kozaruk@ti.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>