dma-buf/fence: Fix one more kerneldoc warning

The seqno_fence_init() function's cond argument isn't described in the
kerneldoc comment. Fix that to silence a warning when building DocBook
documentation.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org>