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: Sangwook Lee <sangwook.lee@linaro.org>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>