Bluetooth: A2DP: Fix mistake parameter

After configuration, the stream instance is valid and stream's
local_ep valid. bt_a2dp_stream_establish's parameter is stream
too. So in bt_a2dp_stream_establish, stream->local_ep should be
used to tell lower level (AVDTP) the sep.
set_config_param is used by copy-paste mistake.

Signed-off-by: Mark Wang <yichang.wang@nxp.com>