Correct SPL use of STM32MP15X_STM32IMAGE

This converts 1 usage of this option to the non-SPL form, since there is
no SPL_STM32MP15X_STM32IMAGE defined in Kconfig

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/arm/mach-stm32mp/fdt.c b/arch/arm/mach-stm32mp/fdt.c
index 3b4c05d..de5c5a5 100644
--- a/arch/arm/mach-stm32mp/fdt.c
+++ b/arch/arm/mach-stm32mp/fdt.c
@@ -505,7 +505,7 @@
 		 * under CONFIG_STM32MP15x_STM32IMAGE only for compatibility
 		 * when FIP is not used by TF-A
 		 */
-		if (CONFIG_IS_ENABLED(STM32MP15x_STM32IMAGE) &&
+		if (IS_ENABLED(CONFIG_STM32MP15x_STM32IMAGE) &&
 		    !tee_find_device(NULL, NULL, NULL, NULL))
 			stm32_fdt_disable_optee(blob);
 	}