IEEE 802.15.4g-2012 specifies a PDU of 2047 for it's PHY modes, so
query the driver for the mode before returning the PDU.
If none of the new modes is used, don't query the driver to not incure
a penalty on existing platforms.
With ace5db70e8 the IID may be gotten from
the device again. However, following the current refactoring efforts
regarding the `netdev_ieee802154` layer, the devices don't necessarily
store the address in the `netdev_ieee802154_t` struct anymore. So we
need to access the address using the driver's `get` function here
instead.