Bluetooth: Audio: Add iso_chan to bt_bap_ep_info

By exposing the underlying iso channel, the application can retrieve
properties of the iso channel.

This can be useful in case the application wants to use the connection
handle for vendor specific HCI commands or to retrieve the particular
parameters used for the given channel.

Signed-off-by: Rubin Gerritsen <rubin.gerritsen@nordicsemi.no>