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>

drivers/bluetooth/btmtkuart.c

@@ -959,16 +959,16 @@ static void btmtkuart_remove(struct serdev_device *serdev)
 	hci_free_dev(hdev);
 }
 
-static const struct btmtkuart_data mt7622_data = {
+static const struct btmtkuart_data mt7622_data __maybe_unused = {
 	.fwname = FIRMWARE_MT7622,
 };
 
-static const struct btmtkuart_data mt7663_data = {
+static const struct btmtkuart_data mt7663_data __maybe_unused = {
 	.flags = BTMTKUART_FLAG_STANDALONE_HW,
 	.fwname = FIRMWARE_MT7663,
 };
 
-static const struct btmtkuart_data mt7668_data = {
+static const struct btmtkuart_data mt7668_data __maybe_unused = {
 	.flags = BTMTKUART_FLAG_STANDALONE_HW,
 	.fwname = FIRMWARE_MT7668,
 };