gprs_gmm: Cancel Location: use detach type IMSI

The detach type network side is defined as
- Reattach required
- Reattach not required
- IMSI detach (after VLR failure)

IMSI detach seems to be more close. Howeever the standard
isn't clear about this.

Change-Id: I27da6dc5165819cccd1ae0a98b132b45a01f38bb
1 file changed