mmc: core: skip stop flow when current request already done

When mmc_stop_request() has no request to stop, because underlying
host driver reported this, the flow should wait for the mmc done()
callback marking that urgent still pending.

CRs-Fixed: 538117
Change-Id: Ib684949619043882df69f4f1f9c45e28a67a5248
Signed-off-by: Konstantin Dorfman <kdorfman@codeaurora.org>