iuup: Add missing state to bitmask for st SMpSDU_Data_Transfer_Ready

The event is expected since the user of the IuUP stack can send a
CONFIGURE.req to transition to state null. The handling was already
there in the function, but the bit was missing in the FSM definition.

Change-Id: I830835a5b8b98f8b91b866f5280f508098c9ae7e
diff --git a/src/gsm/iuup.c b/src/gsm/iuup.c
index 2c70307..bd9e82c 100644
--- a/src/gsm/iuup.c
+++ b/src/gsm/iuup.c
@@ -826,7 +826,8 @@
 		.action = iuup_fsm_init,
 	},
 	[IUUP_FSM_ST_SMpSDU_DATA_XFER_READY] = {
-		.in_event_mask = S(IUUP_FSM_EVT_IUUP_DATA_REQ) |
+		.in_event_mask = S(IUUP_FSM_EVT_IUUP_CONFIG_REQ) |
+				 S(IUUP_FSM_EVT_IUUP_DATA_REQ) |
 				 S(IUUP_FSM_EVT_IUUP_DATA_IND),
 		.out_state_mask = S(IUUP_FSM_ST_NULL) |
 				  S(IUUP_FSM_ST_INIT),