gprs: Fix premature TLLI change
gprs: Fix premature TLLI change

During the GPRS Attach procedure, the SGSN was changing the TLLI
to the new TLLI when sending the second Identity Request.  This
was caused by the sgsn_mm_ctx_by_tlli() function changing the TLLI.
It is usually bad form to have a lookup function change values.

jonathanrsantos/opensgsn,jonathanrsantos/opensgsn