iio:chemical:vz89x: Introduce local struct device pointer.

Avoids lots of repetition of &client->dev and will make the next
patch tidier.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Acked-by: Matt Ranostay <matt.ranostay@konsulko.com>
Link: https://lore.kernel.org/r/20200910173242.621168-29-jic23@kernel.org