iio: adc: ti-ads1015: Deduplicate channel macros

These macros differ only in the number of valid bits of each ADC sample
and the shift of those bits, i.e. ADS1015 is 12bit ADC shifted by 4 left,
ADS1115 is 16bit ADC shifted by 0. No functional change.

Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Andy Shevchenko <andy@kernel.org>
Cc: Daniel Baluta <daniel.baluta@nxp.com>
Link: https://lore.kernel.org/r/20220328194725.149150-4-marex@denx.de
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>