LeAudio: Fix not releasing audio interface

Only the temporary pointer values were set on interface aquisition,
and the original pointers which are then checked when releasing the
interface stayed unset and prevented the release.

This change fixes the following log:
 BTAudioClientLeAudioStub: packages/modules/Bluetooth/system/audio_hal_interface/le_audio_software.cc:103 - Cleanup: HAL transport: 0x01, is broadcast: 0
 bt_stack: [WARNING:le_audio_software.cc(762)] ReleaseSink, can't release not acquired sink

Bug: 320275987
Test: m com.android.btservices
Flag: EXEMPT; trivial change
Change-Id: I9c28afa5f93bbbc9ddf048503d9c5bb9b377138d