wfcam: Fix bug in untested code.

This evening I had a request that the photometric primitives support
the 1.644FeII filter for tonight's WFCAM observing.  However, I recall
making the requested changes (054dab11226c) nearly a year ago.  They
were not installed on UKIRT.  Now I think this was because I had no
test data for 1.644FeII, and may have left it to Watson to try.  As
almost always happens there was a silly bug which would have been
revealed with any filter.  After quickly fixing the error, UKIRT
pipeline is running OK again.