at86rf230: rework state change and start/stop

This patch removes the current synchron state change function and add a
new function for a state assert. Change the start and stop callbacks to
use this new synchron state change behaviour. It's a wrapper around the
async state change function.

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>