iio: adc: ti-ads1015: Replace data_rate with chip data struct ads1015_data

Instead of storing only data_rate in private data, store pointer to the
whole chip data and use the data_rate from chip data throughout the driver.
No functional change. This is done in preparation for switching to
read_avail().

Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Andy Shevchenko <andy@kernel.org>
Cc: Daniel Baluta <daniel.baluta@nxp.com>
Link: https://lore.kernel.org/r/20220328194725.149150-9-marex@denx.de
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>