at86rf230: fix context pointer handling

This patch changes the context pointer to the parameter given one inside
function at86rf230_async_state_change_start. This could occur problem if
context isn't pointed to lp->state.

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>