A2DP set_parameter: return correct status value

The A2DP offload set_parameter code was returning the value
of a temporary variable use to parse the value pairs
instead of the status of the operation.

This mean that an error was wrongly returned when
setting the value of any key that was not
AUDIO_PARAMETER_RECONFIG_A2DP (the last one parsed).

Test: vts-tradefed run commandAndExit vts -m VtsHalAudioV4_0Target
Bug: 111113768
Change-Id: Ic32bdaafb8d048322d1c560f4f4c3deefeba63df
Signed-off-by: Kevin Rocard <krocard@google.com>