fix delayMicroseconds on Teensy-LC at 24 MHz

https://forum.pjrc.com/threads/28932-LC-is-10-9-times-slower-than-T3-1?p=75775&viewfull=1#post75775

teensy3/pins_teensy.c

 #if defined(KINETISL) && F_CPU == 48000000
 	return count * 1000 + ((current * (uint32_t)87381) >> 22);
 #elif defined(KINETISL) && F_CPU == 24000000
-	return count * 1000 + ((current * (uint32_t)174763) >> 23);
+	return count * 1000 + ((current * (uint32_t)174763) >> 21);
 #endif
 	return count * 1000 + current / (F_CPU / 1000000);
 }