le_audio_software: Fix StartSession for HAL 2.1 devices (fix hal2.2)

Without this patch, source audio session cannot be started when HAL 2.1
is used.

Bug: 150670922
Bug: 197297498
Tag: #feature
Sponsor: jpawlowski@
Test: Compile

Change-Id: Id7d066a7c305ce630e6dbd2916bcc7c5a3b82561