Fix SPW fake file detection

See https://trac.openmicroscopy.org.uk/ome/ticket/11330

Using the dataset from the ticket, ```showinf /tmp/mf1.fake/``` should no longer throw an exception, and should consistently show that there are 16 series (2 plates x 2 acquisitions x 2 well rows x 2 well columns).

/cc @bpindelski