Correct SPL use of USB_FUNCTION_FASTBOOT

This converts 1 usage of this option to the non-SPL form, since there is
no SPL_USB_FUNCTION_FASTBOOT defined in Kconfig

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>

cmd/fastboot.c

@@ -44,7 +44,7 @@ static int do_fastboot_usb(int argc, char *const argv[],
 	char *endp;
 	int ret;
 
-	if (!CONFIG_IS_ENABLED(USB_FUNCTION_FASTBOOT)) {
+	if (!IS_ENABLED(CONFIG_USB_FUNCTION_FASTBOOT)) {
 		pr_err("Fastboot USB not enabled\n");
 		return CMD_RET_FAILURE;
 	}