trx: factor out TRXD data submission helper
Extract TRXD data sending into trx_data_submit_msg() and
reuse it from trx_if_send_burst()
This separates transport-specific I/O from message construction
and prepares for alternative backends
No functional changes intended
Change-Id: Ie89ac6efe2f5af0b4cbe5a259a18770b8d2de859
