SIMRecords: If there's no EF_SPN record, fallback to EF_PNN

Turns out some SIM (TMobile US, at least) don't fill in the SPN, but
do insert a PNN record for home network (which in the case of a non-MVNO
are the same. a MVNO should always have the SPN filled anyway).

Change-Id: I18388bb071a49698015c76abefb58798f800946c