Fix frequency calculation from prescale

Use Datasheet 7.3.5 Equation 1 to fix frequency calculation from prescale 
https://cdn-shop.adafruit.com/datasheets/PCA9685.pdf

checked by setting chip frequency to 60Hz
read prescale = 105
hand calculation returned 57.58
computation in C++ shows same 57.58 value

difference between input 60 Hz and reverse calculated frequency of 57.58 is due to limitations of the chip and the 252 possible PWM frequencies. This difference is to be expected.

Author: photodude

Adafruit_PWMServoDriver.cpp

@@ -299,13 +299,10 @@ void Adafruit_PWMServoDriver::writeMicroseconds(uint8_t num, uint16_t Microsecon
   // Read prescale and convert to frequency
   double prescale = Adafruit_PWMServoDriver::readPrescale();
   prescale += 1;
-  // Rounding to nearest number is equal to adding 0,5 and floor to nearest number
-  prescale *= 4096;
-  prescale -= 2048;
-  uint32_t freq = FREQUENCY_CALIBRATED;
-  freq *= prescale; // Calculated PCA9685 chip PWM Frequency
-  freq /= 0.9; // Correct for overshoot in the frequency setting
-
+  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