Bluetooth: btusb: avoid unused function warning

The btusb_rtl_cmd_timeout() function is used inside of an
ifdef, leading to a warning when this part is hidden
from the compiler:

drivers/bluetooth/btusb.c:530:13: error: unused function 'btusb_rtl_cmd_timeout' [-Werror,-Wunused-function]

Use an IS_ENABLED() check instead so the compiler can see
the code and then discard it silently.

Fixes: d7ef0d1e ("Bluetooth: btusb: Use cmd_timeout to reset Realtek device")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>

drivers/bluetooth/btusb.c

@@ -3814,8 +3814,8 @@ static int btusb_probe(struct usb_interface *intf,
 		btusb_check_needs_reset_resume(intf);
 	}
 
-#ifdef CONFIG_BT_HCIBTUSB_RTL
-	if (id->driver_info & BTUSB_REALTEK) {
+	if (IS_ENABLED(CONFIG_BT_HCIBTUSB_RTL) &&
+	    (id->driver_info & BTUSB_REALTEK)) {
 		hdev->setup = btrtl_setup_realtek;
 		hdev->shutdown = btrtl_shutdown_realtek;
 		hdev->cmd_timeout = btusb_rtl_cmd_timeout;
@@ -3826,7 +3826,6 @@ static int btusb_probe(struct usb_interface *intf,
 		 */
 		set_bit(BTUSB_WAKEUP_DISABLE, &data->flags);
 	}
-#endif
 
 	if (id->driver_info & BTUSB_AMP) {
 		/* AMP controllers do not support SCO packets */