SPI: Make sure CS line is active-low

If another user configures the SPI bus as CS active-high, the setting
will stick. It is thus possible to end up in a situation where CS is
forced high during SPI transactions. The solution is to explicitly
disable 'cshigh' before using the SpiDev object.

Author: mrnuke

Adafruit_GPIO/SPI.py

@@ -41,8 +41,9 @@ def __init__(self, port, device, max_speed_hz=500000):
         self._device = spidev.SpiDev()
         self._device.open(port, device)
         self._device.max_speed_hz=max_speed_hz
-        # Default to mode 0.
+        # Default to mode 0, and make sure CS is active low.
         self._device.mode = 0
+        self.cshigh = False
 
     def set_clock_hz(self, hz):
         """Set the speed of the SPI clock in hertz.  Note that not all speeds