drivers/char/n_gsm.c: add missing spin_unlock_irqrestore

Add a spin_unlock_irqrestore missing on the error path.  Converting the
return to break leads to the spin_unlock_irqrestore at the end of the
function.

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

)

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

* spin_lock_irqsave(E1,...);
  <+... when != E1
  if (...) {
    ... when != E1
*   return ...;
  }
  ...+>
* spin_unlock_irqrestore(E1,...);
// </smpl>

Signed-off-by: Julia Lawall <julia@diku.dk>
Cc: Greg Kroah-Hartman <gregkh@suse.de>
Cc: Alan Cox <alan@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>