sdhci: sdhci_stop_request() returns error when no request

There is race between sdhci_tasklet_finish()
and sdhci_stop_request(): when the request is finished before
stop flow called, error returned.

Change-Id: Iac43e6af2498bc75bc8cc959c680a92f9dd5ee7a
Signed-off-by: Konstantin Dorfman <kdorfman@codeaurora.org>