le_audio: Fix using audio location of 0x00

The value of 0x00 is now known as NotAllowed, and should not be used
in case of the mono audio signal.

Bug: 150670922
Tag: #feature
Test: atest --host bluetooth_le_audio_test bluetooth_le_audio_client_test
Sponsor: jpawlowski@
Change-Id: Iab383209a114efa35bf1e3bb1f1b51fce0561624