ath6kl: Fix compilation error from of.h

When compiling ath6kl for ARM with device tree tree compilation fails
with errors like:

include/linux/of.h: In function 'of_property_read_u32_array':
include/linux/of.h:249:10: error: 'ENOSYS' undeclared

Workaround this by including errno.h from init.c.

kvalo: improved commit log
Signed-off-by: NSangwook Lee <sangwook.lee@linaro.org>
Signed-off-by: NKalle Valo <kvalo@qca.qualcomm.com>

drivers/net/wireless/ath/ath6kl/init.c

@@ -16,6 +16,7 @@
  */
 
 #include <linux/moduleparam.h>
+#include <linux/errno.h>
 #include <linux/of.h>
 #include <linux/mmc/sdio_func.h>
 #include "core.h"