osmo-trx-bts: Fix incorrect bts shutdown procedure in case of abis connection closure
osmo-trx-bts: Fix incorrect bts shutdown procedure in case of abis connection closure

This issue occurs in case of osmo-nitb restart which leads to abis connection closure.
Function bts_shutdown from common/bts.c should be used in this case for proper bts shutdown.

Change-Id: Id025e703dd5c91896d450d200e88e46552f178f0

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