scan-for-best-tpr3.rb: Don't fail if we can't get the dcdc3 voltage

This is normal for the devices without AXP209 PMIC (such as the
A13-OLinuXino-MICRO board). So just report dcdc3_vol as 0 if it
is not available.
scan-for-best-tpr3.rb: Remove the finished work item file before reboot

Doing this cleanup as soon as possible is useful when generating html
reports for the partially collected data.