mlx4_core: Clean up error flow in mlx4_register_mac()

Fix a leak of entry if radix_tree_insert() fails.

Also, reduce the indentation and make the flow easier to read by
sticking to the conventional

    err = do_something();
    if (err)
            return err;

    err = do_another();
    if (err)
            return err;

rather than mixing the direction of the test as

    err = do_something();
    if (!err) {
            err = do_another();
            if (err)
                    return err;
    } else
            return err;

Signed-off-by: Roland Dreier <roland@purestorage.com>