tbf_ul_ass_fsm.c: Fix missing state transition in FSM description

As seen operating PCU after BTS restart, lots of following message
sequences due to FSM kept in same state (hence scheduler retyring every
time):
"""
DTBF tbf_ul_ass_fsm.c:306 UL_ASS_TBF(DL-TFI_0){SEND_ASS}: Received Event CREATE_RLCMAC_MSG
DTBF tbf_ul_ass_fsm.c:95 TBF(TFI=0 TLLI=0xf80bd801 DIR=DL STATE=RELEASING EGPRS) We have a schedule for uplink assignment, but there is no uplink TBF
DTBF tbf_ul_ass_fsm.c:97 UL_ASS_TBF(DL-TFI_0){SEND_ASS}: transition to state NONE not permitted!
DTBF tbf_ul_ass_fsm.c:306 UL_ASS_TBF(DL-TFI_0){SEND_ASS}: Received Event CREATE_RLCMAC_MSG
"""

Change-Id: I91d74f70a9106ccbf0c137b6e713877f9ea8f59d

src/tbf_ul_ass_fsm.c

@@ -257,7 +257,9 @@ static struct osmo_fsm_state tbf_ul_ass_fsm_states[] = {
 	},
 	[TBF_UL_ASS_SEND_ASS] = {
 		.in_event_mask = X(TBF_UL_ASS_EV_CREATE_RLCMAC_MSG),
-		.out_state_mask = X(TBF_UL_ASS_WAIT_ACK),
+		.out_state_mask =
+			X(TBF_UL_ASS_WAIT_ACK) |
+			X(TBF_UL_ASS_NONE),
 		.name = "SEND_ASS",
 		.action = st_send_ass,
 	},