isdn: mISDN: remove unneeded variable 'ret'

Fix the following coccicheck warning:
./drivers/isdn/mISDN/dsp_core.c:956:6-9: Unneeded variable: "err".
Return "0" on line 1001
Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Yang Li <yang.lee@linux.alibaba.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/isdn/mISDN/dsp_core.c

@@ -953,7 +953,6 @@ dsp_ctrl(struct mISDNchannel *ch, u_int cmd, void *arg)
 {
 	struct dsp		*dsp = container_of(ch, struct dsp, ch);
 	u_long		flags;
-	int		err = 0;
 
 	if (debug & DEBUG_DSP_CTRL)
 		printk(KERN_DEBUG "%s:(%x)\n", __func__, cmd);
@@ -998,7 +997,7 @@ dsp_ctrl(struct mISDNchannel *ch, u_int cmd, void *arg)
 		module_put(THIS_MODULE);
 		break;
 	}
-	return err;
+	return 0;
 }
 
 static void