iio: adc: ad7192: Fix ac excitation feature

AC excitation enable feature exposed to user on AD7192, allowing a bit
which should be 0 to be set. This feature is specific only to AD7195. AC
excitation attribute moved accordingly.

In the AD7195 documentation, the AC excitation enable bit is on position
22 in the Configuration register. ACX macro changed to match correct
register and bit.

Note that the fix tag is for the commit that moved the driver out of
staging.

Fixes: b581f748

 ("staging: iio: adc: ad7192: move out of staging")
Signed-off-by: Alisa Roman <alisa.roman@analog.com>
Cc: stable@vger.kernel.org
Reviewed-by: Nuno Sa <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20230614155242.160296-1-alisa.roman@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>