Bluetooth: btmtk: Fix btmtk.c undefined reference build error

MediaTek moved some usb interface related function to btmtk.c which
may cause build failed if BT USB Kconfig wasn't enabled.
Fix undefined reference by adding config check.

btmtk.c:(.text+0x89c): undefined reference to `usb_alloc_urb'
btmtk.c:(.text+0x8e3): undefined reference to `usb_free_urb'
btmtk.c:(.text+0x956): undefined reference to `usb_free_urb'
btmtk.c:(.text+0xa0e): undefined reference to `usb_anchor_urb'
btmtk.c:(.text+0xb43): undefined reference to `usb_autopm_get_interface'
btmtk.c:(.text+0xb7e): undefined reference to `usb_autopm_put_interface'
btmtk.c:(.text+0xf70): undefined reference to `usb_disable_autosuspend'
btmtk.c:(.text+0x133a): undefined reference to `usb_control_msg'

Fixes: d019930b ("Bluetooth: btmtk: move btusb_mtk_hci_wmt_sync to btmtk.c")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202407091928.AH0aGZnx-lkp@intel.com/Signed-off-by: Chris Lu <chris.lu@mediatek.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>

drivers/bluetooth/btmtk.c

@@ -437,6 +437,7 @@ int btmtk_process_coredump(struct hci_dev *hdev, struct sk_buff *skb)
 }
 EXPORT_SYMBOL_GPL(btmtk_process_coredump);
 
+#if IS_ENABLED(CONFIG_BT_HCIBTUSB_MTK)
 static void btmtk_usb_wmt_recv(struct urb *urb)
 {
 	struct hci_dev *hdev = urb->context;
@@ -1488,6 +1489,7 @@ int btmtk_usb_shutdown(struct hci_dev *hdev)
 	return 0;
 }
 EXPORT_SYMBOL_GPL(btmtk_usb_shutdown);
+#endif
 
 MODULE_AUTHOR("Sean Wang <sean.wang@mediatek.com>");
 MODULE_AUTHOR("Mark Chen <mark-yw.chen@mediatek.com>");

drivers/bluetooth/btmtk.h

@@ -202,6 +202,7 @@ int btmtk_process_coredump(struct hci_dev *hdev, struct sk_buff *skb);
 void btmtk_fw_get_filename(char *buf, size_t size, u32 dev_id, u32 fw_ver,
 			   u32 fw_flavor);
 
+#if IS_ENABLED(CONFIG_BT_HCIBTUSB_MTK)
 int btmtk_usb_subsys_reset(struct hci_dev *hdev, u32 dev_id);
 
 int btmtk_usb_recv_acl(struct hci_dev *hdev, struct sk_buff *skb);
@@ -216,6 +217,7 @@ int btmtk_usb_suspend(struct hci_dev *hdev);
 int btmtk_usb_setup(struct hci_dev *hdev);
 
 int btmtk_usb_shutdown(struct hci_dev *hdev);
+#endif
 #else
 
 static inline int btmtk_set_bdaddr(struct hci_dev *hdev,