tux3: Replace buffers_entry macro by an inline

As an inline we get more sensible error messages and more type safety.
We should fix more of these, but this one caused actual difficulty and
therefore got fixed first.

Signed-off-by: Daniel Phillips <d.phillips@partner.samsung.com>
Signed-off-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>