rx51_battery: Fix channel number when reading adc value

This issue was introduced in commit 8e2747f0 "power: rx51_battery:
Replace hardcoded channels values."

Original code use channel as argument which was shifted by one in
function. After mentioned commit argument is already shifted so we need to
get index back.

Signed-off-by: Marek Belisko <marek.belisko@open-nandra.com>
Signed-off-by: Anton Vorontsov <anton@enomsg.org>