Instead of calling trans_log_subsys() for each LOG_TRANS() log line, rather
store in trans->log_subsys once on trans_alloc() and use that.
Do not fall back to the RAN's own subsystem (DBSSAP / DIUCS), it makes little
sense and may cause logging to switch subsystems depending on the RAN state.
In trans_log_subsys(), add missing switch cases:
- Log silent call transactions also on CC.
- Log USSD on DMM.
About USSD: we currently have no dedicated USSD logging category. As a result,
after LOG_TRANS() was introduced [1], USSD logged on DBSSAP/DIUCS or DMSC,
depending on whether a RAN was associated with the trans or not. Before that
change, USSD always logged on DMM, so, until we have a separate logging
category for USSD, consistenly use DMM again.
[1] in I2e60964d7a3c06d051debd1c707051a0eb3101ba / ff7074a0c7
Related: coverity CID 198453
Change-Id: I6dfe5b98fb9e884c2dde61d603832dafceb12123
7f85acea9b