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.

cpu/nrf52/radio/nrf802154/nrf802154_radio.c

@@ -548,6 +548,11 @@ void isr_radio(void)
             break;
         case STATE_ACK:
             _state = STATE_IDLE;
+
+            /* We disable the radio to avoid unwanted emmissions (see ERRATA
+             * ID 204, "Switching between TX and RX causes unwanted emissions")
+             */
+            _disable();
             DEBUG("[nrf52840] TX ACK done.")
             _set_ifs_timer(false);
             break;