at86rf230: fix phy settings while sleeping

While in sleep state then we can't access the at86rf2xx registers. This
patch checks if the transceiver is in sleep state before sending spi
messages via regmap. Regmap is used on every driver ops callback except
for receive and xmit handling, but while receive and xmit handling the
phy should not be inside the sleep state.

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