staging:iio:adc:ad7280a: Use a local dev pointer to avoid &spi->dev

We use this a few times already and it is about to get worse, so
introduce a local variable to simplify things.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Marcelo Schmitt <marcelo.schmitt1@gmail.com>
Link: https://lore.kernel.org/r/20220206190328.333093-14-jic23@kernel.org