mmc: sdhci: Simplify sdhci_finish_command() by clearing host->cmd at the start

sdhci_finish_command() is going to set host->cmd to NULL. Simplify the
code by using a local variable to hold host->cmd and set host->cmd to
NULL at the start.

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