at86rf230: increase sleep to off timings

I expierenced when setting channel while sleep mode it didn't changed
the channel inside the hardware registers. Then I got another report of
an user which has similar issues.

I increased the sleep to off state change timing, which is according
at86rf233 at maximum 1000 us. After this change I got no similar effects
again.

I tried another option to wait on AWAKE_END irq, which can be used to
wait until the transceiver is awaked. I tested it and the IRQ took 4
seconds after starting state change. I don't believe it takes 4 seconds
to go into the TRX_OFF state from SLEEP state. The alternative is to
increase the timings which seems to work.

Cc: Oleg Hahm <oliver.hahm@inria.fr>
Signed-off-by: Alexander Aring <aar@pengutronix.de>
Reviewed-by: Stefan <Schmidt&lt;stefan@osg.samsung.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>