The caller of these functions in btusb.c is guarded with an
if(IS_ENABLED()) style check, so dead code is left out, but the
declarations are still needed at compile time:

drivers/bluetooth/btusb.c: In function 'btusb_mtk_reset':
drivers/bluetooth/btusb.c:2705:15: error: implicit declaration of function 'btmtk_usb_subsys_reset' [-Wimplicit-function-declaration]
 2705 |         err = btmtk_usb_subsys_reset(hdev, btmtk_data->dev_id);
      |               ^~~~~~~~~~~~~~~~~~~~~~
drivers/bluetooth/btusb.c: In function 'btusb_send_frame_mtk':
drivers/bluetooth/btusb.c:2720:23: error: implicit declaration of function 'alloc_mtk_intr_urb' [-Wimplicit-function-declaration]
 2720 |                 urb = alloc_mtk_intr_urb(hdev, skb, btusb_tx_complete);
      |                       ^~~~~~~~~~~~~~~~~~
drivers/bluetooth/btusb.c:2720:21: error: assignment to 'struct urb *' from 'int' makes pointer from integer without a cast [-Wint-conversion]
 2720 |                 urb = alloc_mtk_intr_urb(hdev, skb, btusb_tx_complete);
      |                     ^

Fixes: f0c83a23 ("Bluetooth: btmtk: Fix btmtk.c undefined reference build error")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>