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

CONFIG_SPL_FS_SQUASHFS cannot be disabled when CONFIG_FS_SQUASHFS is
enabled. Fix it.

Signed-off-by: Pali Rohár <pali@kernel.org>
diff --git a/fs/fs.c b/fs/fs.c
index 97d4791..b812597 100644
--- a/fs/fs.c
+++ b/fs/fs.c
@@ -309,7 +309,7 @@
 	},
 #endif
 #endif
-#if IS_ENABLED(CONFIG_FS_SQUASHFS)
+#if CONFIG_IS_ENABLED(FS_SQUASHFS)
 	{
 		.fstype = FS_TYPE_SQUASHFS,
 		.name = "squashfs",