Correct SPL use of TPM_RNG

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

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/tpm/tpm-uclass.c b/drivers/tpm/tpm-uclass.c
index 5ff0cd3..b2286f7 100644
--- a/drivers/tpm/tpm-uclass.c
+++ b/drivers/tpm/tpm-uclass.c
@@ -156,7 +156,7 @@
 	const char *drv = TPM_RNG_DRV_NAME;
 	struct udevice *child;
 
-	if (CONFIG_IS_ENABLED(TPM_RNG)) {
+	if (IS_ENABLED(CONFIG_TPM_RNG)) {
 		ret = device_find_first_child_by_uclass(dev, UCLASS_RNG,
 							&child);