mmc: block: Reduce polling timeout from 10 minutes to 10 seconds

Set a 10 second timeout for polling write request busy state. Note, mmc
core is setting a 3 second timeout for SD cards, and SDHCI has long had a
10 second software timer to timeout the whole request, so 10 seconds should
be ample.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Tested-by: Linus Walleij <linus.walleij@linaro.org>