Set DTX in Cell Options

* rename field of struct gsm48_cell_options to better match the spec
* add comments with spec references
* add function for setting DTX in cell options struct
* add necessary enum type

Change-Id: I5a8924f57669c951b2e51b663d95f1d360062a54
Reviewed-on: https://gerrit.osmocom.org/39
Reviewed-by: Harald Welte <laforge@gnumonks.org>
Tested-by: Jenkins Builder
3 files changed