mmc: sdhci: Reorganize sdhci_finish_mrq() and __sdhci_finish_mrq()

In preparation for removing finish_tasklet, reorganize sdhci_finish_mrq()
and __sdhci_finish_mrq() to separate the tasklet scheduling from other
processing.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Reviewed-by: Faiz Abbas <faiz_abbas@ti.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>