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.
at86rf230: rework reset to trx_off state change
at86rf230: add timing for channel switch
at86rf230: add sleep cycle timing
at86rf230: add new author