fs: Allow to compile FS_FAT only for proper U-Boot

CONFIG_SPL_FS_FAT cannot be disabled when CONFIG_FS_FAT is enabled. Fix it.

Signed-off-by: Pali Rohár <pali@kernel.org>
diff --git a/fs/fs.c b/fs/fs.c
index c3a2ed9..2b0a242 100644
--- a/fs/fs.c
+++ b/fs/fs.c
@@ -177,7 +177,7 @@
 };
 
 static struct fstype_info fstypes[] = {
-#ifdef CONFIG_FS_FAT
+#if CONFIG_IS_ENABLED(FS_FAT)
 	{
 		.fstype = FS_TYPE_FAT,
 		.name = "fat",