Minor change to convertfpM function to correctly propagate SDSS mask planes

In the convertfpM(infile, allPlanes=False) function the condition where allPlanes=True
was not well tested.  This takes the additional SDSS fpM mask planes and makes an
equivalent plane for LSST.  The name propagation was incorrect (an additoinal underscore
was left in) and the values were set incorrectly (the mask plane Id was being used, not
2**Id).  This is a minor change not used by production code so am committing on master.