board: stm32mp1: solve compilation issue when ENV_IS_IN_MMC is deactivated

Solve compilation issue on undefined CONFIG_SYS_MMC_ENV_DEV when
CONFIG_ENV_IS_IN_MMC is deactivated on STMicroelectronics boards
defconfig

Fixes: 9f97193616f1 ("board: stm32mp1: use CONFIG_SYS_MMC_ENV_DEV when available")
Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>
diff --git a/board/st/stm32mp1/stm32mp1.c b/board/st/stm32mp1/stm32mp1.c
index 45f2ca8..fff1880 100644
--- a/board/st/stm32mp1/stm32mp1.c
+++ b/board/st/stm32mp1/stm32mp1.c
@@ -890,8 +890,10 @@
 
 int mmc_get_env_dev(void)
 {
-	if (CONFIG_SYS_MMC_ENV_DEV >= 0)
-		return CONFIG_SYS_MMC_ENV_DEV;
+	const int mmc_env_dev = CONFIG_IS_ENABLED(ENV_IS_IN_MMC, (CONFIG_SYS_MMC_ENV_DEV), (-1));
+
+	if (mmc_env_dev >= 0)
+		return mmc_env_dev;
 
 	/* use boot instance to select the correct mmc device identifier */
 	return mmc_get_boot();