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/block.c b/drivers/mmc/card/block.c
index fb387c8..adf0ed3 100644
--- a/drivers/mmc/card/block.c
+++ b/drivers/mmc/card/block.c
@@ -259,7 +259,7 @@
 	u32 result;
 	__be32 *blocks;
 
-	struct mmc_request mrq;
+	struct mmc_request mrq = {0};
 	struct mmc_command cmd = {0};
 	struct mmc_data data = {0};
 	unsigned int timeout_us;
@@ -300,8 +300,6 @@
 	data.sg = &sg;
 	data.sg_len = 1;
 
-	memset(&mrq, 0, sizeof(struct mmc_request));
-
 	mrq.cmd = &cmd;
 	mrq.data = &data;