llc: Mark old/current tlli as all 1's when unassigning LLME

TS 44.064 section 8.3.3 (and other sections) talk about special
unassigned value of "all 1's", but I couldn't find any reference to a
"all 0's" specific value/meaning.
In practice in the code this may not be super important since those
values may not ve checked due to the FSM state, but in any case they are
initially set to all 1's, so it makes total sense to re-set them to the
same unassigned value instead of a randomly chosen all 0's value.

Change-Id: I660c8d0ef08b34f8cb74fd51b5c59e5628d687ae

src/sgsn/gprs_llc.c

@@ -1112,7 +1112,7 @@ int gprs_llgmm_assign(struct gprs_llc_llme *llme,
 		llme->state = GPRS_LLMS_ASSIGNED;
 	} else if (old_tlli != TLLI_UNASSIGNED && new_tlli == TLLI_UNASSIGNED) {
 		/* TLLI Unassignment 8.3.3) */
-		llme->tlli = llme->old_tlli = 0;
+		llme->tlli = llme->old_tlli = TLLI_UNASSIGNED;
 		llme->state = GPRS_LLMS_UNASSIGNED;
 		for (i = 0; i < ARRAY_SIZE(llme->lle); i++) {
 			struct gprs_llc_lle *l = &llme->lle[i];