correct SS pin setup is already handled by SPI subsystem.
this should prevent future issues like #2868
current implementation assures that:
* pin10 is OUTPUT HIGH if SPI.begin() is called and the pin was unconfigured
* pin10 state is not modified if pinMode(10, OUTPUT) is called before SPI.begin()
* pin10 is INPUT HI-Z if nor pinMode(10, OUTPUT) nor SPI.begin() are called