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/spi.h

@@ -3,6 +3,6 @@
 
 #include <stdint.h>
 
-void spi_write_byte(uint8_t);
+uint8_t spi_byte(uint8_t);
 
 #endif