gprs_gmm: Fix missing Security Command for 3G when attaching

Introduce a new FSM step in GMM Attach to send the
Security Command to the RNC after completing the
Authentication.

Fixes: f7198d7dbb84 ("gprs_gmm: introduce a GMM Attach Request FSM")
Change-Id: I1e12b0a32e58c6f78dba7b548f7d7016567229db
3 files changed