nrf802154/hal: fix unwanted emissions at the end of ACK frame
nrf802154/hal: fix unwanted emissions at the end of ACK frame

This commit turns off the transceiver immediately after sending an ACK
frame in order to avoid unwanted emissions (see ERRATA ID 204).
When the radio stays in TX_ON without an explicit transition, the radio
keeps sending a spurious signal for some milliseconds.

miri64/RIOT,OTAkeys/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,OlegHahm/RIOT,OlegHahm/RIOT,kYc0o/RIOT,authmillenon/RIOT,OlegHahm/RIOT,jasonatran/RIOT,OTAkeys/RIOT,OlegHahm/RIOT,jasonatran/RIOT,OTAkeys/RIOT,miri64/RIOT,miri64/RIOT,OTAkeys/RIOT,miri64/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,authmillenon/RIOT,jasonatran/RIOT,kaspar030/RIOT,kaspar030/RIOT,jasonatran/RIOT,OlegHahm/RIOT,ant9000/RIOT,kYc0o/RIOT,kYc0o/RIOT,authmillenon/RIOT,ant9000/RIOT,kaspar030/RIOT,OTAkeys/RIOT,jasonatran/RIOT,kYc0o/RIOT,ant9000/RIOT,ant9000/RIOT,authmillenon/RIOT,ant9000/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,miri64/RIOT,RIOT-OS/RIOT