mmc: sdhci: Avoid STOP cmd triggering warning in sdhci_send_command()

The STOP command is sent in error conditions, even when the command is
not finished. Avoid triggering the warning for that in sdhci_send_command()
by setting host->cmd to NULL first.

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