bts: revert trx shutdown order
bts: revert trx shutdown order

When a new TRX is allocated using gsm_bts_trx_alloc() (see gsm_data_shared.c in
openbsc.git), than it is added to the list in order. When octphy is shutting
down the BTS, it uses llist_for_each_entry() to iterate the tansceiver list to
shut all transceivers down. This means it starts the shut down process with
the primary TRX and then continues with the secondary transceivers in order.

However, octphy does not allow to close primary TRX if the secondary TRX is
open. The shutdown sequence must begin with the secondary transceivers and
finish with the primiary transceiver as last item.

The problem can be easily fixed by iterating the transceiver list in reverse
order using llist_for_each_entry_reverse() instead of llist_for_each_entry()

Since this is a change in the common code, all BTS models (not only octphy)
are affected, but from the logical perspective, this change makes sense
for all other BTS models too.

Change-Id: I18485de586b402610f9e98053d69e92bc9b18dc2

osmocom/osmo-bts,osmocom/osmo-bts,osmocom/osmo-bts