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>
diff --git a/drivers/mmc/card/mmc_test.c b/drivers/mmc/card/mmc_test.c
index f44762b..35252c1 100644
--- a/drivers/mmc/card/mmc_test.c
+++ b/drivers/mmc/card/mmc_test.c
@@ -246,15 +246,13 @@
 {
 	int ret;
 
-	struct mmc_request mrq;
+	struct mmc_request mrq = {0};
 	struct mmc_command cmd = {0};
 	struct mmc_command stop = {0};
 	struct mmc_data data = {0};
 
 	struct scatterlist sg;
 
-	memset(&mrq, 0, sizeof(struct mmc_request));
-
 	mrq.cmd = &cmd;
 	mrq.data = &data;
 	mrq.stop = &stop;
@@ -728,13 +726,11 @@
 	struct scatterlist *sg, unsigned sg_len, unsigned dev_addr,
 	unsigned blocks, unsigned blksz, int write)
 {
-	struct mmc_request mrq;
+	struct mmc_request mrq = {0};
 	struct mmc_command cmd = {0};
 	struct mmc_command stop = {0};
 	struct mmc_data data = {0};
 
-	memset(&mrq, 0, sizeof(struct mmc_request));
-
 	mrq.cmd = &cmd;
 	mrq.data = &data;
 	mrq.stop = &stop;
@@ -755,15 +751,13 @@
 static int mmc_test_broken_transfer(struct mmc_test_card *test,
 	unsigned blocks, unsigned blksz, int write)
 {
-	struct mmc_request mrq;
+	struct mmc_request mrq = {0};
 	struct mmc_command cmd = {0};
 	struct mmc_command stop = {0};
 	struct mmc_data data = {0};
 
 	struct scatterlist sg;
 
-	memset(&mrq, 0, sizeof(struct mmc_request));
-
 	mrq.cmd = &cmd;
 	mrq.data = &data;
 	mrq.stop = &stop;