canfdtest: fix typo while waiting for tv_stop time

After a few complaint about 100% CPU usage of canfdtest over our PCI
-like boards, I have had a look to the source file, and think I have
found the issue in can_echo_dut().

Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

canfdtest.c

@@ -239,7 +239,7 @@ static int can_echo_dut(void)
 				}
 				gettimeofday(&tvn, NULL);
 				while ((tv_stop.tv_sec > tvn.tv_sec) ||
-				       ((tv_stop.tv_sec = tvn.tv_sec) &&
+				       ((tv_stop.tv_sec == tvn.tv_sec) &&
 					(tv_stop.tv_usec >= tvn.tv_usec)))
 					gettimeofday(&tvn, NULL);
 			}