at86rf230: remove hrtimer on 1 usec delay

According Documentation/timers/timers-howto.txt the usually case for
setting up a hrtimer takes > ~10us. So we should use udelay in this
case so we are sure that the state change was done, before doing the
state change assert.

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>