drivers: adc: adc_ad559x: fix 5593 adc read

The AD5593 conversion result also contains the channel read and not just a
value:
| adc channel (3 bits) | adc value (12 bits) |

This value was not removed from the result before using it as an adc value.

Reuse the AD5592 code to fix the issue.

Signed-off-by: Jeppe Odgaard <jeppe.odgaard@prevas.dk>