Simplify and improve the writeMicroseconds() calculations

Checked the calculations by hand

Use Datasheet 7.3.5 Equation 1 for frequency from prescale
https://cdn-shop.adafruit.com/datasheets/PCA9685.pdf
Frequency = (ClockFrequency/(PreScale+1))*(1/BitRate)
and from the rest of the calculation
us per period = PulseLength/Frequency
us per bit = us per period/BitRate
 we can combine to get
us per bit = (PulseLength/((ClockFrequency/(PreScale+1))*(1/BitRate)))*(1/BitRate)

This can be simplified to (PulseLength (PreScale + 1))/ClockFrequency 
this formula was implemented in proposed code

checked by setting chip frequency to 60Hz
read prescale = 105
us per bit = 4.069 with 25Mhz (a PWM frequency of about 57.3hz)
Choosing the FREQUENCY_CALIBRATED constant listed at 104.3% higher or 26.075MHz
we get 
us per bit = 4.1035 or a PWM frequency of about 60.056Hz much closer to the set 60Hz 

Overall this change makes less calculation steps, and uses fewer variables. The change should be faster and more accurate to the intended results.

Author: photodude

Adafruit_PWMServoDriver.cpp

@@ -296,24 +296,14 @@ void Adafruit_PWMServoDriver::writeMicroseconds(uint8_t num, uint16_t Microsecon
   double pulselength;
   pulselength = 1000000;   // 1,000,000 us per second
 
-  // Read prescale and convert to frequency
+  // Read prescale
   double prescale = Adafruit_PWMServoDriver::readPrescale();
-  prescale += 1;
-  uint32_t freq = 25000000; // Chip frequency is 25MHz
-  freq /= prescale;
-  freq /= 4096; // 12 bits of resolution
- 
-  #ifdef ENABLE_DEBUG_OUTPUT
-  Serial.print(freq); Serial.println(" Calculated PCA9685 chip PWM Frequency");
-  #endif
-  
-  pulselength /= freq;   //  us per period from PCA9685 chip PWM Frequency using prescale reverse frequency calc
+  Serial.print(prescale); Serial.println(" PCA9685 chip prescale");
 
-  #ifdef ENABLE_DEBUG_OUTPUT
-  Serial.print(pulselength); Serial.println(" us per period");
-  #endif
-
-  pulselength /= 4096;  // 12 bits of resolution
+  // Calculate the pulse for PWM based on Equation 1 from the datasheet section 7.3.5
+  prescale += 1;
+  pulselength *= prescale; 
+  pulselength /= FREQUENCY_CALIBRATED;
 
   #ifdef ENABLE_DEBUG_OUTPUT
   Serial.print(pulselength); Serial.println(" us per bit");