Bluetooth: Host: Modify check for read buffer size v2

Modify the check from checking the feature bit to
checking the command bit. This ensures that we
don't send the read buffer size V2 to a controller
that does not support it.

This also moves the entire ISO init procedure into
a separate function to avoid having a large
ISO-only block in `le_init`.

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