drivers: adc_common: Add missing gain values in adc_gain_invert()

This is a follow-up to commits e15bdaa1
and 07bf22cc.

The above two commits added some enumerated gain values and those
are not currently handled by the inverting function. Add the missing
entries to the conversion array in the function.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>