navigator: usual usleep after poll fail

We don't want to busy loop after the poll fail if that ever happened.

src/modules/navigator/navigator_main.cpp

@@ -349,6 +349,7 @@ Navigator::task_main()
 		} else if (pret < 0) {
 			/* this is undesirable but not much we can do - might want to flag unhappy status */
 			PX4_WARN("nav: poll error %d, %d", pret, errno);
+			usleep(10000);
 			continue;
 		} else {