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/mmc_test.c b/drivers/mmc/card/mmc_test.c
index 95c298f..f44762b 100644
--- a/drivers/mmc/card/mmc_test.c
+++ b/drivers/mmc/card/mmc_test.c
@@ -249,12 +249,11 @@
 	struct mmc_request mrq;
 	struct mmc_command cmd = {0};
 	struct mmc_command stop = {0};
-	struct mmc_data data;
+	struct mmc_data data = {0};
 
 	struct scatterlist sg;
 
 	memset(&mrq, 0, sizeof(struct mmc_request));
-	memset(&data, 0, sizeof(struct mmc_data));
 
 	mrq.cmd = &cmd;
 	mrq.data = &data;
@@ -732,10 +731,9 @@
 	struct mmc_request mrq;
 	struct mmc_command cmd = {0};
 	struct mmc_command stop = {0};
-	struct mmc_data data;
+	struct mmc_data data = {0};
 
 	memset(&mrq, 0, sizeof(struct mmc_request));
-	memset(&data, 0, sizeof(struct mmc_data));
 
 	mrq.cmd = &cmd;
 	mrq.data = &data;
@@ -760,12 +758,11 @@
 	struct mmc_request mrq;
 	struct mmc_command cmd = {0};
 	struct mmc_command stop = {0};
-	struct mmc_data data;
+	struct mmc_data data = {0};
 
 	struct scatterlist sg;
 
 	memset(&mrq, 0, sizeof(struct mmc_request));
-	memset(&data, 0, sizeof(struct mmc_data));
 
 	mrq.cmd = &cmd;
 	mrq.data = &data;