fs: relax ext4_write_file() dependency

ext4_write_file() depends on CONFIG_EXT4_WRITE. Allow build without
CONFIG_CMD_EXT4_WRITE.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
diff --git a/fs/fs.c b/fs/fs.c
index bed1f72..0c47943 100644
--- a/fs/fs.c
+++ b/fs/fs.c
@@ -224,7 +224,7 @@
 		.exists = ext4fs_exists,
 		.size = ext4fs_size,
 		.read = ext4_read_file,
-#ifdef CONFIG_CMD_EXT4_WRITE
+#ifdef CONFIG_EXT4_WRITE
 		.write = ext4_write_file,
 		.ln = ext4fs_create_link,
 #else