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: NTom Rini <trini@konsulko.com>

fs/fat/fat.c

@@ -54,7 +54,7 @@ static int disk_read(__u32 block, __u32 nr_blocks, void *buf)
 
 	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;