Merge pull request arduino#692 from Khosos/patch-1

Update I2S.cpp to work with RTCZero library

Author: facchinm

libraries/I2S/src/I2S.cpp

@@ -422,6 +422,7 @@ void I2SClass::enableClock(int divider)
   GCLK->GENCTRL.bit.ID = _clockGenerator;
   GCLK->GENCTRL.bit.SRC = src;
   GCLK->GENCTRL.bit.IDC = 1;
+  GCLK->GENCTRL.bit.DIVSEL = 0;
   GCLK->GENCTRL.bit.GENEN = 1;
 
   // enable