mmc: Remove unused variable backup from mmc_send_cmd()

Do not call a memset for unused variable backup every time.
Remove unused variable from function.

Signed-off-by: Oleksandr Tyshchenko <oleksandr.tyshchenko@ti.com>
Acked-by: Pantelis Antoniou <panto@antoniou-consulting.com>

drivers/mmc/mmc.c

@@ -55,11 +55,8 @@ int board_mmc_getcd(struct mmc *mmc)__attribute__((weak,
 static int mmc_send_cmd(struct mmc *mmc, struct mmc_cmd *cmd,
 			struct mmc_data *data)
 {
-	struct mmc_data backup;
 	int ret;
 
-	memset(&backup, 0, sizeof(backup));
-
 #ifdef CONFIG_MMC_TRACE
 	int i;
 	u8 *ptr;