at86rf230: remove unnecessary spinlock

This spinlock isn't necessary because if we are in TX_ON/TX_ARET_ON
state the transceiver can't be interrupted e.g. by receiving a frame
when a SHR was detected. In this time the transceiver doesn't leave
the TX_ON/TX_ARET_ON state until the tx complete irq change the state
into RX_AACK_ON again. This means a receiving interrupt in state
TX_ON/TX_ARET_ON can't happen and is_tx is protected by transceiver.

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