Bluetooth: Audio: Fix mcc.c `conn` logging statements

mcc.c does not include `conn_internal.h` so it does not
know the `bt_conn` struct, causing logging issues when
trying to log a `bt_conn` pointer.

Fixed by casting the `conn` pointers to `(void *)`.

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