mmc: sdhci: sdhci_execute_tuning() must delete timer

sdhci_send_command() starts a timer to catch cases where the host
controller fails. The timer is normally deleted when the request completes,
but in the case of sdhci_execute_tuning() the request is handled
differently and the timer is left running. This goes unnoticed because
tuning is done before another command so the timer gets reset then.
That should not be relied upon, so make sdhci_execute_tuning() delete the
timer.

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