drivers: adc: gecko: Fix error return values

Enable the adc_error_cases test on Series 2 devices and change
the return values in the driver for unsupported configurations
to what the test expects.

Ensure that the resolution configuration in the test is valid
such that the invalid buffer test correctly receives -ENOMEM.

Signed-off-by: Aksel Skauge Mellbye <aksel.mellbye@silabs.com>