proto_trxd: add helpers to prepare burst indication packets
- introduce trxd_prepare_burst_ind_v0() and trxd_prepare_burst_ind_v1()
- reuse prepared packet size in send helpers
- handle idle frames during packet preparation
- use ssize_t consistently for prepared/write sizes
Change-Id: Ie94a1df4879cbf6056cd05c9621c46db1ed972fd
