ekf2_main: reduce stack (according to the reductions in the commit before)

src/modules/ekf2/ekf2_main.cpp

@@ -1044,7 +1044,7 @@ int Ekf2::start()
 	_control_task = px4_task_spawn_cmd("ekf2",
 					   SCHED_DEFAULT,
 					   SCHED_PRIORITY_MAX - 5,
-					   6000,
+					   5800,
 					   (px4_main_t)&Ekf2::task_main_trampoline,
 					   nullptr);