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>
diff --git a/cmd/fastboot.c b/cmd/fastboot.c
index 17c53bb..97dc02c 100644
--- a/cmd/fastboot.c
+++ b/cmd/fastboot.c
@@ -44,7 +44,7 @@
 	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;
 	}