rsl: rename, fix and refactor lchan_tchmode_from_cmode()
rsl: rename, fix and refactor lchan_tchmode_from_cmode()

In change [1] I added the missing 'default' branch to the 'switch'
statement in lchan_tchmode_from_cmode().  This caused massive
regressions in ttcn3-bts-test, because osmo-bts started to NACK
some RSL CHANnel ACTIVation messages.

What caused a lot of regressions in ttcn3-bts-test is actually the
missing branch for RSL_CMOD_SPD_SIGN in the 'switch' statement.
It was not a problem before [1], because the 'default' branch was
not there.  I was about to add the missing 'cause' when I realized
that this function needs to be reworked first...

First of all, lchan_tchmode_from_cmode() does a bit more than just
deriving RR (TS 44.018) channel mode from RSL (TS 48.058) channel
mode.  It additionally stores the 'Speech or data indicator' to
the logical channel state, and also changes some global DTXd related
flags in 'struct gsm_bts'.  Let's use a more precise name.

  lchan_tchmode_from_cmode() -> rsl_handle_chan_mod_ie()

Together with renaming, it becomes logical to have the IE presence
check in rsl_handle_chan_mod_ie(), so that we can reduce code
duplication in the calling functions a bit.

Finally, the main problem is that coding and interpretation of the
6-th octet 'Speech coding algor./data rate + transp ind' depends on
the 4-th octet of the Channel Mode IE.  We cannot handle all values
in one 'switch' statement without proper discrimination:

  a) If octet 4 indicates Speech, then octet 6 shall be interpreted
     as the GSM speech coding algorithm (FR, HR, AMR, etc.).

  b) If octet 4 indicates Signalling, then octet 6 shall be set
     to '00'O, because this is the only value defined in version
     16.0.0 of 3GPP TS 48.058.  All other values are reserved.

  c) If octet 4 indicates Data, then octet 6 shall be interpreted
     as CSD data rate further discriminated by service transparency.

Therefore, we need take both values into account.  This can be
achieved by mixing them together using the bitwise operators,
just like we do in L1SAP code.

Change-Id: Iba967f5bd0cc8ad6cd3ccd40cca38b15ffe96b2c
Related: [1] I67a70132999be6580a29e6b814763309a6df4ae9
Related: SYS#4895, OS#4941

osmocom/osmo-bts,osmocom/osmo-bts,osmocom/osmo-bts