at86rf230: remove unnecessary tx state change

All supported transceivers can do a valid state change from TRX_OFF to
AACK_ON. This patch removes the state change chain from TRX_OFF ->
TX_ON -> AACK_ON instead we doing a directly state change from TRX_OFF
to AACK_ON.

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