IB/mlx4: Convert is_xxx variables in build_mlx_header() to bool

Matches the way they're used, and actually lets at least x86-64 generate
better code:

    add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-38 (-38)
    function                                     old     new   delta
    mlx4_ib_post_send                           4416    4378     -38

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