Bluetooth: Audio: Move ISO struct from audio_endpoint

Moves the bt_iso_chan and bt_iso_chan_qos structs
out from the bt_audio_ep struct, and instead use a
reference.

This is due to the fact that an endpoint can ever
only be unidirectional, and a CIS can be bidrectional.

The support for using a single bididrectional CIS
for two audio streams/endpoints is not implemented
in this commit.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>