leaudio: Fix getting first device.

In case weak pointer is expired, we should check if there is another
valid one

Sponsor: jpawlowski@
Bug: 150670922
Test: atest --host bluetooth_le_audio_test
bluetooth_le_audio_client_test

Change-Id: Iec45914a56d53f1b08742d027e8b1f0d4a3b0d0a