at86rf230: remove tx_timeout

This patch removes tx_timeout handling. We used it in sync xmit
handling. Since we support async xmit handling a xmit timeout handling
isn't easy to implement and should be implemented by netdev watchdog
mechanism.

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