mmc: initialize struct mmc_request at declaration time

Converts from:
	struct mmc_request mrq;
	memset(&mrq, 0, sizeof(struct mmc_request));

to:
	struct mmc_request mrq = {0};

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

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