cc2538: spi: Include spi-arch.h before dev/spi.h

spi-arch.h configures dev/spi.h, so it must be #included first. Luckily, this
mistake did not have any consequence here, but fix it in order to avoid possible
future issues.

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>

cpu/cc2538/dev/spi.c

@@ -35,12 +35,12 @@
  */
 #include "contiki.h"
 #include "reg.h"
+#include "spi-arch.h"
 #include "dev/ioc.h"
 #include "dev/sys-ctrl.h"
 #include "dev/spi.h"
 #include "dev/ssi.h"
 #include "dev/gpio.h"
-#include "spi-arch.h"
 
 /**
  * \brief Initialize the SPI bus.