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>