Make sure SPI backend is initialized in SPIFlash driver

In some cases, SPIFlash driver hangs during initialization
because it reads data from the SPI backend. But if SPI backend
has not been initialized, this fails.

libraries/MySensors/utility/SPIFlash.cpp

@@ -56,6 +56,7 @@ void SPIFlash::unselect() {
 /// setup SPI, read device ID etc...
 boolean SPIFlash::initialize()
 {
+  SPI.begin();
   _SPCR = SPCR;
   _SPSR = SPSR;
   pinMode(_slaveSelectPin, OUTPUT);