SGSN: Fix RAN information exposed on GTP during PDP CTX CREATE

In the PDP Context Create from SGSN to GGSN, we include information
about the RAN type (GERAN/UTRAN) and the Cell of the MS. This was
all hard-coded to GERAN, and wasn't updated when we added UTRAN
support to the SGSN.

Change-Id: I6c79e42c5e08b28fe8182555302a5505fbbaa313