generic-adc-battery: Fix checking if none of the channels are supported

If none of the channels are supported, index is 0. Also ensure to return
error code instead of 0 in goto second_mem_fail path.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: anish kumar <anish198519851985@gmail.com>
Signed-off-by: Anton Vorontsov <anton@enomsg.org>