Bluetooth: hci_bcm: Store device pointer instead of platform_device pointer

The ACPI subsys is going to move over to instantiating ACPI enumerated
HCIs as serdevs, rather then as platform devices.

This means that the serdev driver paths of hci_bcm.c also need to start
supporting (runtime)pm through GPIOs and a host-wake IRQ.

The hci_bcm code is already mostly independent of how the HCI gets
instantiated, but even though the code only cares about pdev->dev, it
was storing pdev itself in struct bcm_device.

This commit stores pdev->dev rather then pdev in struct bcm_device, this
is a preparation patch for adding (runtime)pm support to the serdev path.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>