mmc: cqhci: allow hosts to update dcmd cmd desc

This patch adds update_dcmd_desc interface to cqhci_host_ops to
allow hosts to update any of the DCMD task descriptor attributes
and parameters.

Tested-by: Jon Hunter <jonathanh@nvidia.com>
Reviewed-by: Ritesh Harjani <riteshh@codeaurora.org>
Signed-off-by: Sowjanya Komatineni <skomatineni@nvidia.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>