mmc: initialize struct mmc_command at declaration time

Converts from:
	struct mmc_command cmd;
	memset(&cmd, 0, sizeof(struct mmc_command));

to:
	struct mmc_command cmd = {0};

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

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