Bluetooth: btmtk: hide unused btmtk_mt6639_devs[] array

Bluetooth: btmtk: hide unused btmtk_mt6639_devs[] array

When USB support is disabled, the array is not referenced anywhere,
causing a warning:

drivers/bluetooth/btmtk.c:35:3: error: 'btmtk_mt6639_devs' defined but not used [-Werror=unused-const-variable=]
   35 | } btmtk_mt6639_devs[] = {
      |   ^~~~~~~~~~~~~~~~~

Move it into the #ifdef block.

Fixes: 28b7c5a6db74 ("Bluetooth: btmtk: Add MT6639 (MT7927) Bluetooth support")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Paul Menzel <pmenzel@molgen.mpg.de>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>