kw2xrf: Fetch short addr from device

This change modifies the kw2xrf_get_addr_short function to retrieve the
short address from the device and not from netdev

drivers/kw2xrf/kw2xrf_getset.c

@@ -293,7 +293,11 @@ void kw2xrf_set_addr_long(kw2xrf_t *dev, uint64_t addr)
 
 uint16_t kw2xrf_get_addr_short(kw2xrf_t *dev)
 {
-    return (dev->netdev.short_addr[0] << 8) | dev->netdev.short_addr[1];
+    uint16_t addr;
+    uint8_t *ap = (uint8_t *)(&addr);
+    kw2xrf_read_iregs(dev, MKW2XDMI_MACSHORTADDRS0_LSB, ap,
+                      IEEE802154_SHORT_ADDRESS_LEN);
+    return byteorder_swaps(addr);
 }
 
 uint64_t kw2xrf_get_addr_long(kw2xrf_t *dev)