mesh: LPN non-low power establishment set state

Sets the LPN state to WAIT_UPDATE when waiting for the initial poll
response when disabling LPN_ESTABLISHMENT. Previously, the LPN node
would stay in the wait offer state even after the offer was processed,
which led to it aborting the friend establishment completely if it
failed to receive the first poll.

This is port of 2e3de6c5f828b5eef136f4090e3af1d41e6654e9
diff --git a/nimble/host/mesh/src/lpn.c b/nimble/host/mesh/src/lpn.c
index 988d426..60ca843 100644
--- a/nimble/host/mesh/src/lpn.c
+++ b/nimble/host/mesh/src/lpn.c
@@ -344,6 +344,7 @@
 		k_delayed_work_submit(&lpn->timer,
 				      LPN_RECV_DELAY - SCAN_LATENCY);
 	} else {
+		lpn_set_state(BT_MESH_LPN_WAIT_UPDATE);
 		k_delayed_work_submit(&lpn->timer,
 				      LPN_RECV_DELAY + duration +
 				      lpn->recv_win);