Changes

E1 BTS: direct MGW to 8k subslots for TCH/H

If the design of an E1 BTS allocates a 16 kbit/s subslot on Abis
for each GSM timeslot, then if that GSM ts is configured to operate
as two TCH/H lchans instead of a single TCH/F lchan, that BTS has
to use the 8 kbit/s TRAU frame format of GSM 08.61 and not the
16 kbit/s format for each TCH/H lchan.  Nokia E1 BTS and their
associated BSC and TCSM (TRAU) equipment are known to use this
arrangement; the present author does not know of any GSM BSS vendor
who used the 16 kbit/s format of GSM 08.61 - it might be a paper-only
spec.

Prior to the present change OsmoBSC always selected a 16k subslot
endpoint at the MGW, thus precluding possibility of working TCH/H
on otherwise supported E1 BTS.  Change it to select 8k subslot
endpoints when operating on TCH/H lchans.

It may seem that this change breaks support for those E1 BTS that
do use 16 kbit/s TRAU frame format for TCH/H, whatever they may be -
but in reality, said support was already broken.  Prior to the
present change OsmoBSC would smoosh both TCH/H lchans of a given
GSM ts onto the same MGW endpoint - which cannot work because they
are two different lchans carrying unrelated traffic.

Change-Id: If6141a55434ce08f8fdd1c69b6fc9a97d6c726a7