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: Julia Lawall <julia@diku.dk>
Signed-off-by: David S. Miller <davem@davemloft.net>