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 <[e-mail]>
Signed-off-by: Luiz Augusto von Dentz <[e-mail]>

drivers/bluetooth/btmtkuart.c

···
 	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,
 };