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>
diff --git a/drivers/mmc/card/block.c b/drivers/mmc/card/block.c
index 93a7efc..fb387c8 100644
--- a/drivers/mmc/card/block.c
+++ b/drivers/mmc/card/block.c
@@ -261,7 +261,7 @@
 
 	struct mmc_request mrq;
 	struct mmc_command cmd = {0};
-	struct mmc_data data;
+	struct mmc_data data = {0};
 	unsigned int timeout_us;
 
 	struct scatterlist sg;
@@ -282,8 +282,6 @@
 	cmd.arg = 0;
 	cmd.flags = MMC_RSP_SPI_R1 | MMC_RSP_R1 | MMC_CMD_ADTC;
 
-	memset(&data, 0, sizeof(struct mmc_data));
-
 	data.timeout_ns = card->csd.tacc_ns * 100;
 	data.timeout_clks = card->csd.tacc_clks * 100;