[le audio] Fix the null pointer crash in SetAllowedDsaModes

Add null pointer check for before calling interface pointer.

Bug: 315069812
Bug: 309665975
Test: atest bluetooth_le_audio_client_test bluetooth_test_broadcaster bluetooth_le_audio_test
Test: manual check starting broadcast should not crash
Change-Id: If5088bdf17de42f6389942d19adbe8aa3ca8e4fe