mmc: mmc: Attempt to flush cache before reset

CMD0 or hardware reset may invalidate the cache, so it needs to be
flushed before reset.

In the case of recovery, we can't expect flushing the cache to work
always, but have a go and ignore errors.

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