spi: Make Spi write() work from SWIG with typemaps

This change also changes the C++ API write(char) call to writeByte(uint8_t) and
the write() call now takes a uint8_t* instead of a char*. This should not alter
any code significantly and does not affect the C API.

Signed-off-by: Brendan Le Foll <brendan.le.foll@intel.com>

examples/c++/Spi-pot.cpp

@@ -48,9 +48,9 @@ int main ()
 
     spi = new mraa::Spi(0);
 
-    char data[] = {0x00, 100};
-    char rxBuf[2];
-    char *recv;
+    uint8_t data[] = {0x00, 100};
+    uint8_t rxBuf[2];
+    uint8_t *recv;
     while (running == 0) {
         int i;
         for (i = 90; i < 130; i++) {