audio_a2dp_hw: fix fd leak on open

Open hal fds can be checked by

adb shell lsof | grep audio@2. | grep socket | wc

Test: connect and disconnect BT, check open fd.
Bug: 37726871

Change-Id: Id446c43515eceb0828cfb33807a4c0b4c6145a6d
(cherry picked from commit cef156574c9928164b7a5e263a5494cf61ca39d8)