linux: Convert linux SPI code to use the generic spicmds.c code

Use the generic spi send/receive code on Linux.  Update the replicape
code to use the updated command format.

Also, update the replicape code to turn off the stepper motors on a
shutdown event.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

src/linux/Kconfig

@@ -7,6 +7,7 @@ config LINUX_SELECT
     bool
     default y
     select HAVE_GPIO_ADC
+    select HAVE_GPIO_SPI
 
 config BOARD_DIRECTORY
     string