at86rf230: use STATE_TX_ARET mode only

This patch changes the state change behaviour of at86rf230 to always
TX_ARET mode. According the at86rf2xx datasheets TX_ARET mode doesn't
mean to be always waiting for ack frames after transmit. The transceiver
will automatically wait for ack frames or not if the acknowledge request
bit is set. See section "TX_ARET_ON – Transmit with Automatic Frame
Retransmission and CSMA-CA Retry".

Reviewed-by: Stefan Schmidt <stefan@osg.samsung.com>
Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>