Bluetooth: ISO: Remove bt_iso_data_path struct

The struct basically only had a pointer to
bt_iso_chan_path as well as duplicating the pid and
the direction.

The commit removes the struct as it was more confusing than helpful,
and instead use the PID for the bt_iso_chan_path and
supply the direction as a argument instead.

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