Bluetooth: host: Use correct user_data size for hci_rx_pool

`struct acl_data` is used even when Host flow control is not enabled.
It is written to through the `acl(buf)` accessor in `conn.c:hci_acl()`.

Hopefully no netbufs were harmed by that :/

Signed-off-by: Jonathan Rico <jonathan.rico@nordicsemi.no>