Change spi_write_byte to spi_byte

the SPI module reads and writes at the same time. If you don't want to
write a value, then write all zeros or whatever the device you are
communicating with wants.

sw/tests/test_main.c

@@ -1,10 +1,26 @@
 #include <stdio.h>
+#include <stdint.h>
+
 #include <spi.h>
 
+char retval;
+
 int main(void)
 {
-    printf("Starting spi_write_byte test...\n");
-    spi_write_byte(0xa5);
-    printf("Done!\n");
+    printf("Setting SPI location to 0x02\n");
+    *(uint8_t*)0x7ff0 = 2;
+    if (!(*(uint8_t*)0x7ff0 == 2)) {
+        printf("Expected 0x02 at 0x7ff0\n");
+        return 1;
+    }
+    printf("Done!\n\n");
+
+    printf("Starting spi_byte test...\n");
+    retval = spi_byte(0xa5);
+    if (retval != 0) {
+        printf("Expected 0 return value from spi_byte\n");
+        return 1;
+    }
+    printf("Done!   %x\n", retval);
     return 0;
 }