drivers/isdn/hardware/mISDN: Add missing spin_unlock

Add a spin_unlock missing on the error path.  The return value of write_reg
seems to be completely ignored, so it seems that the lock should be
released in every case.

The semantic match that finds this problem is as follows:
(http://coccinelle.lip6.fr/)

// <smpl>
@@
expression E1;
@@

* spin_lock(E1,...);
  <+... when != E1
  if (...) {
    ... when != E1
*   return ...;
  }
  ...+>
* spin_unlock(E1,...);
// </smpl>
Signed-off-by: NJulia Lawall <julia@diku.dk>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/isdn/hardware/mISDN/hfcsusb.c

@@ -97,8 +97,10 @@ static int write_reg(struct hfcsusb *hw, __u8 reg, __u8 val)
 			hw->name, __func__, reg, val);
 
 	spin_lock(&hw->ctrl_lock);
-	if (hw->ctrl_cnt >= HFC_CTRL_BUFSIZE)
+	if (hw->ctrl_cnt >= HFC_CTRL_BUFSIZE) {
+		spin_unlock(&hw->ctrl_lock);
 		return 1;
+	}
 	buf = &hw->ctrl_buff[hw->ctrl_in_idx];
 	buf->hfcs_reg = reg;
 	buf->reg_val = val;