mmc: sdhci: Allow for finishing multiple requests

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
recording which request is finished.  Doing that obsoletes host->mrq which
is therefore removed.

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