Bluetooth: Use proper datatypes in release-callbacks

This enhances code readability a lot and avoids using void* even though
we know the type of the variable.

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>