mmc: sdhci: Clear pointers when a request finishes

Several pointers are used to identify when interrupts are expected. Namely,
host->cmd, host->data_cmd and host->data. Ensure those are cleared when
a request finishes. That tidies the case when a request is errored out
before normal processing has completed, ensuring any interrupts that occur
subsequently are not acted upon.

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