mlx4_core: Fix advertisement of wrong PF context behaviour

Commit 08ff3235

 ("mlx4: 64-byte CQE/EQE support") introduced a
regression where older guest VF drivers failed to load even when
64-byte EQEs/CQEs are disabled, since the PF wrongly advertises the
new context behaviour anyway.  The failure looks like:

    mlx4_core 0000:00:07.0: Unknown pf context behaviour
    mlx4_core 0000:00:07.0: Failed to obtain slave caps
    mlx4_core: probe of 0000:00:07.0 failed with error -38

Fix this by basing this advertisement on dev->caps.flags, which is the
operational capabilities used by the QUERY_FUNC_CAP command wrapper
(dev_cap->flags holds the firmware capabilities).

Reported-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>