at86rf230: fix is_tx while error handling

This patch fix the error handling when is_tx is true. The error handling
tries to get the transceiver into RX_AACK_ON mode then we need to be
sure that is_tx is false.

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