Bluetooth: coredump: fix building with coredump disabled

The btmtk driver uses an IS_ENABLED() check to conditionally compile
the coredump support, but this fails to build because the hdev->dump
member is in an #ifdef:

drivers/bluetooth/btmtk.c: In function 'btmtk_process_coredump':
drivers/bluetooth/btmtk.c:386:30: error: 'struct hci_dev' has no member named 'dump'
  386 |   schedule_delayed_work(&hdev->dump.dump_timeout,
      |                              ^~

The struct member doesn't really make a huge difference in the total size,
so just remove the #ifdef around it to avoid adding similar checks
around each user.

Fixes: 872f8c253cb9e ("Bluetooth: btusb: mediatek: add MediaTek devcoredump support")
Fixes: 9695ef87 ("Bluetooth: Add support for hci devcoredump")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>