We can only support up to 30 UEs for SRS. In this commit, we change:
- if the total number of UEs is > 32, throw error during compilation to
  raise awareness (32 so that a multiple of two still works, everything
  else is confusing, see also next point)
- during runtime, handle gracefully if UID is 30 or 31 to not abort (we
  can simply not allocate SRS resources)

Also, refactor config_srs() to always give back new memory; the code is
shorter and clearer. Rename to get_contig_srs() to make it explicit.