Power down the field before sending the USB ACK

This fixes the bug that the USB communication breaks when collecting PACE nonces with no delay. The delay functionality has not been removed (it doesn't hurt to keep it), but using a delay of 0 is now possible without a high risk of getting too many errors.

armsrc/epa.c

@@ -217,12 +217,12 @@ static void EPA_PACE_Collect_Nonce_Abort(UsbCommand *ack, uint8_t step, int func
 	ack->arg[0] = step;
 	// last return code
 	ack->arg[1] = func_return;
-	
-	// send the USB packet
-	UsbSendPacket((void *)ack, sizeof(UsbCommand));
 
 	// power down the field
 	EPA_Finish();
+	
+	// send the USB packet
+	UsbSendPacket((void *)ack, sizeof(UsbCommand));
 }
 
 //-----------------------------------------------------------------------------