Improve RPi host kernel panic mitigation

It was still possible to cause a Kernel panic, this additional 500ms delay before disconnect appears to mitigate it completely.

Marlin/src/HAL/LPC1768/HAL.cpp

@@ -64,8 +64,9 @@ int16_t PARSED_PIN_INDEX(const char code, const int16_t dval) {
 }
 
 void flashFirmware(const int16_t) {
+  delay(500);          // Give OS time to disconnect
   USB_Connect(false);  // USB clear connection
-  delay(2000);         // Give OS time to notice
+  delay(1000);         // Give OS time to notice
   NVIC_SystemReset();
 }