Change SDSS pin number for Printrboard.

Like the fan, the code in Marlin does not seem to use fastio to communicate with the SD Card, so the pin numbering is the same as the Teensy++2.0. Fastio, for whatever reason, scrambles the IO pin numbers. See https://github.com/ErikZalm/Marlin/issues/112.

The SD Chip Select is connected to Port B pin 6, which is pin 26 in Teensyduino-land. 

For some reason, this pin was set to 8, which isn't B6 in either Teensyduinoland or Fastio-mangled-Teensyduinoland.