bluetooth: a2dp: optimize bt_a2dp_ep_info

bt_a2dp_ep_info is used in bt_a2dp_discover, and the max
discovered endpoints' info are already given in seps_info
of bt_a2dp_discover_param, so use pointer to point to the
seps_info of bt_a2dp_discover_param.

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