Changes

GTP: CreatePDPContext: only use IMEISV IE when IMEISV is known

The IE is optional, but if it is present, it must be 11 byte long
containing a 8 byte IMEISV. If IMEI is unknown the SGSN
would add an empty IMEISV IE which is invalid.

Change-Id: I812af1e702e77214244f32ae65663c1a03b23962