Use IRQ_PRIORITY (not hard-coded number) in Serial3

teensy3/serial3.c

 	UART2_C1 = 0;
 	UART2_PFIFO = 0;
 	UART2_C2 = C2_TX_INACTIVE;
-	NVIC_SET_PRIORITY(IRQ_UART2_STATUS, 64);
+	NVIC_SET_PRIORITY(IRQ_UART2_STATUS, IRQ_PRIORITY);
 	NVIC_ENABLE_IRQ(IRQ_UART2_STATUS);
 }