Bluetooth: btmtkuart: mark OF related data as maybe unused

The driver can be compile tested with !CONFIG_OF making certain data
unused:

  drivers/bluetooth/btmtkuart.c:971:36: error: ‘mt7668_data’ defined
  but not used [-Werror=unused-const-variable=]
  drivers/bluetooth/btmtkuart.c:966:36: error: ‘mt7663_data’ defined
  but not used [-Werror=unused-const-variable=]
  drivers/bluetooth/btmtkuart.c:962:36: error: ‘mt7622_data’ defined
  but not used [-Werror=unused-const-variable=]

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>