iio: adc: ti-ads1015: write config register only on change

There is no point writing ADS1015_CFG_REG when configuration
didn't change. Avoid that.

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