drivers/at86rf2xx: prevent a possible race condition after state change

It was pointed out that after a state change to RX_AACK_ON reading back
the state to confirm the transition can fail due to an imidiate change
into BUSY_RX_AACK between the successful change on the transceiver and
querying the state.
For this we exclude the readback of the state for transitions to
RX_AACK_ON.

drivers/at86rf2xx/at86rf2xx_getset.c

@@ -421,7 +421,16 @@ void at86rf2xx_set_option(at86rf2xx_t *dev, uint16_t option, bool state)
 static inline void _set_state(at86rf2xx_t *dev, uint8_t state)
 {
     at86rf2xx_reg_write(dev, AT86RF2XX_REG__TRX_STATE, state);
-    while (at86rf2xx_get_status(dev) != state);
+
+    /* To prevent a possible race condition when changing to
+     * RX_AACK_ON state the state doesn't get read back in that
+     * case. See discussion
+     * in https://github.com/RIOT-OS/RIOT/pull/5244
+     */
+    if (state != AT86RF2XX_STATE_RX_AACK_ON) {
+        while (at86rf2xx_get_status(dev) != state);
+    }
+
     dev->state = state;
 }