fs/fat: Correct blk_dread() return value check

The function blk_dread will return -ENOSYS on failure or on success the
number of blocks read, which must be the number asked to read (otherwise
it failed somewhere).  Correct this check.

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/fs/fat/fat.c b/fs/fat/fat.c
index 9ad18f9..465a687 100644
--- a/fs/fat/fat.c
+++ b/fs/fat/fat.c
@@ -54,7 +54,7 @@
 
 	ret = blk_dread(cur_dev, cur_part_info.start + block, nr_blocks, buf);
 
-	if (nr_blocks && ret == 0)
+	if (ret != nr_blocks)
 		return -1;
 
 	return ret;