Bluetooth: hci_bcm: fix build error without CONFIG_PM

This was introduced by the rework adding PM support:

drivers/bluetooth/hci_bcm.c: In function 'bcm_device_exists':
drivers/bluetooth/hci_bcm.c:156:22: error: 'struct bcm_device' has no member named 'hu'
  if (device && device->hu && device->hu->serdev)
                      ^~

The pointer is not available otherwise, so I'm enclosing
all references in an #ifdef here.

Fixes: 8a920568

 ("Bluetooth: hci_bcm: Add (runtime)pm support to the serdev driver")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>