mmc: fix rmmod race for hosts using card-detection polling

MMC hosts that poll for card detection by defining the MMC_CAP_NEEDS_POLL
flag have a race on rmmod, where the delayed work is cancelled without
waiting for completed polling. To prevent this a _sync version of the work
cancellation has to be used.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Cc: <stable@kernel.org>
Signed-off-by: Chris Ball <cjb@laptop.org>