iio:ti-ads1015: fix a wrong pointer definition.

The call to i2c_get_clientdata(client) returns a struct iio_dev*, not
the needed struct ads1015_data*. We need here an intermediate step as
in the function: void ads1015_get_channels_config(struct i2c_client *client).

Signed-off-by: Giorgio Dal Molin <giorgio.nicole@arcor.de>
Fixes: ecc24e72 ("iio: adc: Add TI ADS1015 ADC driver support")
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>