iio: adc: ti-ads1015: Use correct pga upper bound

The devicetree binding and datasheets (for both the ADS1015 and
ADS1115) show that the PGA index should have a maximum value of 5,
and not 6.

Signed-off-by: Mohammed Billoo <mab.kernel@gmail.com>
Link: https://lore.kernel.org/r/20240106174836.1086714-1-mab.kernel@gmail.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>