Extend the delay ensure post reset pulse delayed.

Given the original poster's comment that "It happens very consistently for us." I suspect the motor spin observed in https://github.com/PX4/Firmware/issues/7457 is not caused by the original issue of slow decay on the PWM pins at reset, but the post reset pulse of 3.1 Ms arriving in a window that the ESC considers it valid.

The results from testing, indicated that the if the PWM pins were clamped low for > 300 Ms, prior to reset the motors did not spin. This would delay the the post reset pulse of 3.1 Ms out by > 300 Ms. 

This change delays the reset and therefore the pulse by at least 400 Ms.

src/drivers/boards/px4fmu-v4/px4fmu_init.c

@@ -179,7 +179,7 @@ __EXPORT void board_on_reset(int status)
 	 */
 
 	if (status >= 0) {
-		up_mdelay(6);
+		up_mdelay(400);
 	}
 }