SPI: fix bug in csPin data type

The data type of the csPin member of spi_device_config was wrong.
The field will be set to -1 if no pin is set or manual cs-pin control is used.
Then the comparisons with -1 in the code will not work as expected, see
https://stackoverflow.com/questions/36770376/comparing-unsigned-char-to-a-negative-number

So we use "char" instead of "byte" and the code behaves as expected.

src/SpiFirmata.h

@@ -40,7 +40,7 @@
 struct spi_device_config {
   byte deviceIdChannel;
   byte csPinOptions;
-  byte csPin;
+  char csPin;
   boolean packedData;
   SPISettings spi_settings;
   boolean used;