Merge pull request #168 from KurtE/SPI_SETTINGS_PROTECTED

Allow a sketch to sub-class the SSD1306 object and change SPI settings

Adafruit_SSD1306.h

@@ -176,14 +176,16 @@ class Adafruit_SSD1306 : public Adafruit_GFX {
   PortReg     *mosiPort   , *clkPort   , *dcPort   , *csPort;
   PortMask     mosiPinMask,  clkPinMask,  dcPinMask,  csPinMask;
 #endif
-#if defined(SPI_HAS_TRANSACTION)
-  SPISettings  spiSettings;
-#endif
 #if ARDUINO >= 157
   uint32_t     wireClk;    // Wire speed for SSD1306 transfers
   uint32_t     restoreClk; // Wire speed following SSD1306 transfers
 #endif
   uint8_t      contrast;    // normal contrast setting for this device
+#if defined(SPI_HAS_TRANSACTION)
+protected:
+  // Allow sub-class to change
+  SPISettings  spiSettings;
+#endif
 };
 
 #endif // _Adafruit_SSD1306_H_