ieee802154: return PDU based on PHY mode
ieee802154: return PDU based on PHY mode

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.

ant9000/RIOT,authmillenon/RIOT,basilfx/RIOT,jasonatran/RIOT,authmillenon/RIOT,basilfx/RIOT,miri64/RIOT,ant9000/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,miri64/RIOT,kYc0o/RIOT,OTAkeys/RIOT,OlegHahm/RIOT,OlegHahm/RIOT,authmillenon/RIOT,basilfx/RIOT,OlegHahm/RIOT,jasonatran/RIOT,jasonatran/RIOT,kYc0o/RIOT,basilfx/RIOT,jasonatran/RIOT,basilfx/RIOT,RIOT-OS/RIOT,ant9000/RIOT,miri64/RIOT,kaspar030/RIOT,OlegHahm/RIOT,kYc0o/RIOT,ant9000/RIOT,kaspar030/RIOT,kaspar030/RIOT,OTAkeys/RIOT,OlegHahm/RIOT,RIOT-OS/RIOT,ant9000/RIOT,kaspar030/RIOT,kYc0o/RIOT,kYc0o/RIOT,miri64/RIOT,kaspar030/RIOT,OTAkeys/RIOT,OTAkeys/RIOT,authmillenon/RIOT,OTAkeys/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,miri64/RIOT