iio: potentiometer: tpl0102: switch to using pointer to chip config

More concise to have a pointer to tpl0102_cfg struct in the iio_priv
data than an integer to an index of an array.

Signed-off-by: Matt Ranostay <matt.ranostay@konsulko.com>
Reviewed-by: Slawomir Stepien <sst@poczta.fm>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>