at86rf230: fix race condition

When the driver waits for a tx completion currently the driver direct
enables the irq. When we switching to RX_AACK_ON some steps afterwards
the driver could receive a new frame and request resources which are
already in use, for example irq state change resource.

To be sure there are no new interrupts when we switching to RX_AACK_ON,
we enable the irq when state change to RX_AACK_ON was completed.

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