powersupply.py: Fix import module line typo

Recent commit changed the import line so it runs only when required, but
added a typo making tests uing it fail.

Change-Id: Id3e5c16adc763a9b79cc464937c541abd15f98de
Fixes: 2b959580b9affe729e966c506656b7f47926c4bf
1 file changed