mmc: make mmc_dev_to_card() macro public

Conversion from struct device to struct mmc_card is used more than in one
place.  Due to this it's better to have public macro for such thing.

Signed-off-by: Andy Shevchenko <ext-andriy.shevchenko@nokia.com>
Cc: Adrian Hunter <adrian.hunter@nokia.com>
Cc: <linux-mmc@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Chris Ball <cjb@laptop.org>