bts: Properly free bts->shutdown_fi when struct gsm_bts is freed
bts: Properly free bts->shutdown_fi when struct gsm_bts is freed

Change-Id: I8c59f030c39c83a91b8504d4c5ce3b2058206fcf

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