at86rf230: make at86rf230_async_error inline

This patch makes the at86rf230_async_error inline. This function is
small enough to handle inline.

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Reviewed-by: Stefan Schmidt <s.schmidt@samsung.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>