Changes

gsm48_cc_tx_setup_set_transaction_id: split out

Move the code for setting the transaction ID to a separate function to
start making gsm48_cc_tx_setup shorter and easier to read. Return -1 on
error instead of rc from mncc_release_ind() as it returns 0 on success.

Make some tweaks while at it:
* Set LOGL_ERROR for the "TX Setup with assigned transaction" error.
* Add a log message for the "could not get a free transaction ID" case.
* Minor formatting tweaks.

Change-Id: I715f12d09570211a98b667c56960309bd326e8d8