Add necessary delays to SPI DigitalPotControl example

According to arduino/Arduino#6395, these delays are required for the code to work.

Author: per1234

libraries/SPI/examples/DigitalPotControl/DigitalPotControl.ino

@@ -63,9 +63,11 @@ void loop() {
 void digitalPotWrite(int address, int value) {
   // take the SS pin low to select the chip:
   digitalWrite(slaveSelectPin, LOW);
+  delay(100);
   //  send in the address and value via SPI:
   SPI.transfer(address);
   SPI.transfer(value);
+  delay(100);
   // take the SS pin high to de-select the chip:
   digitalWrite(slaveSelectPin, HIGH);
 }