regulator/bq24022: fix bug in is_enabled function

This seems to be fallout from last October's regulator core rework.
It got noticed only because of recent regulator framework changes.

Signed-off-by: Philipp Zabel <philipp.zabel@gmail.com>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>