drivers: adc: fix ref_internal for ads1x1x
drivers: adc: fix ref_internal for ads1x1x

The ADS1x13 does not have a PGA, and will have a fixed
internal reference voltage of 2048mV. This sets the internal
referece voltage for the ads1x1x to 2048 and adjust the gain
configuration to scale.

Signed-off-by: Ryan McClelland <8b67290475031698c81d147ec4ac7ae4d9451537@fb.com>

galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,galak/zephyr