net: ppp: Reset flag is_ipcp_up in ipcp_down

Without this fix is it not possible to terminate pppd and restart
pppd afterwards without restarting the firmware, too.

Signed-off-by: Christian Taedcke <christian.taedcke@lemonbeat.com>

subsys/net/l2/ppp/ipcp.c

@@ -404,6 +404,7 @@ static void ipcp_down(struct ppp_fsm *fsm)
 	}
 
 	ctx->is_network_up = false;
+	ctx->is_ipcp_up = false;
 
 	ppp_network_down(ctx, PPP_IP);
 }