mmc: delayed_work was never cancelled

The delayed work item mmc_host.detect is now cancelled before flushing
the work queue. This takes care of cases when delayed_work was scheduled
for mmc_host.detect, but not yet placed in the work queue.

Signed-off-by: Jorg Schummer <ext-jorg.2.schummer@nokia.com>
Signed-off-by: Pierre Ossman <drzeus@drzeus.cx>