gsm_data: make cgi_for_msc available for other modules

The function cgi_for_msc() provides an easy way to get a cell global id
for an msc/bts combination. This function is currently statically
defined in gsm_08_08.c. Lets move it to gsm_data.c and make it publicly
available.

Change-Id: I301fac6e83a429ae59b5c586aa283ad7ba54053d
Related: OS#3645
3 files changed