Reset a2dp_cmd_pending as appropriate for A2DP HW offload HAL commands

Previously, the a2dp_cmd_pending was set to the last pending command,
but was never reset to A2DP_CTRL_CMD_NONE after the command processing
was completed.

Bug: 110712574
Test: Manual: Pair with two headsets, start/suspend/stop/switch active
      device
Change-Id: I464e6e9d41a221df44eba21e538b499a9ac1dcfb