gprs_gmm: introduce a GMM Attach Request FSM

The old GMM Attach Request handling used a recursive function
which can not handle certain states and is quite complex and hard to
extend.

The new FSM handles such request in a FSM and can be called multiple
times.

Change-Id: I58b9c17be9776a03bb2a5b21e99135cfefc8c912
1 file changed