iio: adc: xilinx: use helper variable for &pdev->dev

It's more elegant to use a helper local variable to store the address
of the underlying struct device than to dereference pdev everywhere.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Tested-by: Anand Ashok Dumbre <anandash@xilinx.com>
Reviewed-by: Anand Ashok Dumbre <anandash@xilinx.com>
Link: https://lore.kernel.org/r/20201130142759.28216-2-brgl@bgdev.pl
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>