common/oml.c: use proper OML object for Get Attribute Response
common/oml.c: use proper OML object for Get Attribute Response

It was noticed that the Get Attribute Response message always
indicates BTS(00,ff,ff) as the addressed OML entity, even if
e.g. Baseband Transceiver(00,00,ff) was requested by the BSC.

Despite neither OsmoBSC nor OpenBSC does complain about this,
such behaviour violates 3GPP TS 52.021. Let's fix this.

Change-Id: Icb1ee75d4bf680deb6365288d8c2053816a12217
Related: OS#3938

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