drivers/at86rf2xx: improve precondition checks on state transition

The rational behind this change is the following:
If the transceiver is in any *_BUSY state when `at86rf2xx_set_state()`
gets called this would bypass the `(state == old_state)` check and
unneeded state transitions could be triggered.

drivers/at86rf2xx/at86rf2xx_getset.c

@@ -429,9 +429,6 @@ void at86rf2xx_set_state(at86rf2xx_t *dev, uint8_t state)
 {
     uint8_t old_state = at86rf2xx_get_status(dev);
 
-    if (state == old_state) {
-        return;
-    }
     /* make sure there is no ongoing transmission, or state transition already
      * in progress */
     while (old_state == AT86RF2XX_STATE_BUSY_RX_AACK ||
@@ -440,6 +437,10 @@ void at86rf2xx_set_state(at86rf2xx_t *dev, uint8_t state)
         old_state = at86rf2xx_get_status(dev);
     }
 
+    if (state == old_state) {
+        return;
+    }
+
     /* we need to go via PLL_ON if we are moving between RX_AACK_ON <-> TX_ARET_ON */
     if ((old_state == AT86RF2XX_STATE_RX_AACK_ON &&
              state == AT86RF2XX_STATE_TX_ARET_ON) ||