iio: adc: ti-ads1015: remove unnecessary config register update

iio: adc: ti-ads1015: remove unnecessary config register update

The ti-ads1015 driver has eight iio voltage channels and each iio channel
can hold own scale information.

The ADS1015 device only have a single config register which contains an
input multiplexer selection, PGA and data rate settings.  So the driver
should load the correct settings when the input multiplexer selection is
changed.

However, regardless of which channlel is currently selected, changing any
iio channel's scale information immediately overwrites the current PGA
setting in the config register.

It is harmless because the correct PGA settings are reloaded just before
getting adc result anyway.  But it is unnecessary register update and
should be removed.

Cc: Daniel Baluta <daniel.baluta@gmail.com>
Cc: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>