MMC: convert bus code to use dev_groups

The dev_attrs field of struct bus_type is going away soon, dev_groups
should be used instead.  This converts the MMC bus code to use the
correct field.

Cc: Chris Ball <cjb@laptop.org>
Cc: Ulf Hansson <ulf.hansson@linaro.org>
Cc: Konstantin Dorfman <kdorfman@codeaurora.org>
Cc: Seungwon Jeon <tgih.jun@samsung.com>
Cc: <linux-mmc@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>