Bluetooth: Introduce to_hci_conn

This avoids using the dev_set/get_drvdata() functions to retrieve a
pointer to our own structure. We can use simple pointer arithmetic here.
The drvdata field is actually not needed by any other code-path but this
makes the code more consistent with hci_dev.

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>