Fix A2DP audio playback problem

With some A2DP headphones/car kits an issue was observed where the SBC
codec could not be initialized correctly and thus no audio was heard
over A2DP. This change fixes a variable initialization issue that
prevented the request for capabilities from being sent to the remote
device.

Change-Id: Ic55da0dbe5abd1e5bb89c8305b0a4c1f06b60a65