mac802154: Adds ackreq flag to TX descriptor for use by radio driver.

The radio layer is responsible for a full "transaction". Because of
differences in radio capabilites, the radio must be responsible for
the handling of acks and retransmissions. This commit simply passes
the ackreq meta-data flag along to the radio.

Approved-by: Gregory Nutt <gnutt@nuttx.org>

ieee802154 MAC: Free allocated TX descriptor if an error occurs after the descriptor has been allocated.

ieee802154_req_data:  Don't modify the IOB until we are certain that no EINTR errors will occur.  Otherwise, the retry will fail

Swithches IEEE 802.15.4 based code to using byte arrays instead of uint16_t values for short address and PAN ID