Changes

silent-call: add CSD channel modes

This is useful for BER testing and figuring out if the baseband
supports the requested channel mode, e.g. half-rate data calls.

Going below 2.4k (0.6k, 1.2k, 1.2k_75) makes no sense, because:

* these rates are carried by TCH/[FH]2.4 channels anyway, and
* we never tell the user data rate to the MS (part of CC SETUP)

The same applies to transparent vs non-transparent channel modes,
but let's add non-transparent variants anyway, just in case the
BTS PHY does not support transparent modes.

Change-Id: Ife83f5bcb2b68a228a014f47c7cbb21660290593