Remove spi_word and replace with spi_deselect

Since the goal is to have the MAX3421E working, which uses multiple 8
bit transfers, we should support multiple 8 bit transfers instead of
trying to use 16 bit transfers.

When using spi_byte(), the device is automatically selected. After you
have made as many transfers as you want, you must deselect the device
with spi_deselect().

sw/spi.h

@@ -4,6 +4,6 @@
 #include <stdint.h>
 
 uint8_t spi_byte(uint8_t);
-uint16_t spi_word(uint16_t);
+void spi_deselect(void);
 
 #endif