mmc: sdhci: Factor out sdhci_finish_mrq()

In order to support commands during data transfer, there will have to be up
to two active requests (mrqs) at a time, instead of just one. That means
the driver must identify which one to finish. Prepare for that by factoring
out sdhci_finish_mrq().

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