dma-buf: fence-chain: Document missing dma_fence_chain_init() parameter in kerneldoc

Fix W=1 compile warnings (invalid kerneldoc):

    drivers/dma-buf/dma-fence-chain.c:233: warning: Function parameter or member 'seqno' not described in 'dma_fence_chain_init'

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200819175134.19261-2-krzk@kernel.org