oml: Only shut the bts down once
oml: Only shut the bts down once

If the shutdown timer is already running do not deactivate the RF and
do not close the trx. This is addressing another instance of the following
warning:

[ERROR] : DeviceMng_ValidateL1Handle() => Invalid layer 1 handle

geosphere/osmo-bts,shimaore/osmo-bts,telenoobie/osmo-bts,osmocom/osmo-bts,geosphere/osmo-bts,shimaore/osmo-bts,geosphere/osmo-bts,telenoobie/osmo-bts,geosphere/osmo-bts,shimaore/osmo-bts,shimaore/osmo-bts,telenoobie/osmo-bts,osmocom/osmo-bts,osmocom/osmo-bts,telenoobie/osmo-bts