Fix coding conventions in sys/transceiver/transceiver.c
Allow IEEE 802.15.4 addressing types for transceiver

IEEE 802.15.4 has two addressing modes: 16-bit short and the device's
EUI-64. Currently RIOT supports only sending of packets with 16-bit
short addresses via the transceiver interface. This patch allows at
least for the radio chips that support IEEE 802.15.4 to let the
application/upper layer decide which addressing mode to use.

Upper layer implementation will be implemented in follow-up PR to #460