Fix IntervalTimer on Teensy LC

teensy3/IntervalTimer.cpp

 void IntervalTimer::end() {
 	if (channel) {
 		int index = channel - KINETISK_PIT_CHANNELS;
+#if defined(KINETISK)
 		NVIC_DISABLE_IRQ(IRQ_PIT_CH0 + index);
+#elif defined(KINETISL)
+		// TODO: disable IRQ_PIT, but only if both instances ended
+#endif
 		funct_table[index] = dummy_funct;
 		channel->TCTRL = 0;
 #if defined(KINETISL)