iio: adc: ad799x: use local device pointer in probe

iio: adc: ad799x: use local device pointer in probe

Introduce a local device pointer 'dev' in ad799x_probe() and use it
throughout the function instead of accessing &client->dev repeatedly.

Suggested-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Reviewed-by: David Lechner <dlechner@baylibre.com>
Signed-off-by: Archit Anant <architanant5@gmail.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>