mmc: sdhci: Reduce the use of host->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
host->mrq will not be able to be used.

In several places, host->mrq is used when instead the mrq can be determined
from the cmd or data pointers. Reduce the use of host->mrq by doing that.

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