mmc: fix integer assignments to pointer

Fix the sparse warning output "warning: Using plain integer as NULL pointer"

Signed-off-by: Venkatraman S <svenkatr@ti.com>
Signed-off-by: Chris Ball <cjb@laptop.org>
diff --git a/drivers/mmc/core/core.c b/drivers/mmc/core/core.c
index eb3069d..557856b 100644
--- a/drivers/mmc/core/core.c
+++ b/drivers/mmc/core/core.c
@@ -371,7 +371,7 @@
  */
 int mmc_wait_for_cmd(struct mmc_host *host, struct mmc_command *cmd, int retries)
 {
-	struct mmc_request mrq = {0};
+	struct mmc_request mrq = {NULL};
 
 	WARN_ON(!host->claimed);