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>