serial: arm_dcc: Fix compilation warning and remove unneeded initialization

- arm_dcc_dev is already initialized.
- Remove unused rc variable
Warning log:
arm_dcc.c: In function 'drv_arm_dcc_init':
arm_dcc.c:145:6: warning: unused variable 'rc' [-Wunused-variable]

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Acked-by: Marek Vasut <marex@denx.de>

drivers/serial/arm_dcc.c

@@ -142,11 +142,6 @@ static struct stdio_dev arm_dcc_dev;
 
 int drv_arm_dcc_init(void)
 {
-	int rc;
-
-	/* Device initialization */
-	memset(&arm_dcc_dev, 0, sizeof(arm_dcc_dev));
-
 	strcpy(arm_dcc_dev.name, "dcc");
 	arm_dcc_dev.ext = 0;	/* No extensions */
 	arm_dcc_dev.flags = DEV_FLAGS_INPUT | DEV_FLAGS_OUTPUT;