mmc: initialize struct mmc_data at declaration time

Converts from:
	struct mmc_data data;
	memset(&data, 0, sizeof(struct mmc_data));

to:
	struct mmc_data data = {0};

because it's shorter, as performant, and easier to work out whether
initialization has happened.

Signed-off-by: Chris Ball <cjb@laptop.org>