arch/powerpc/platforms/83xx/mpc837x_mds.c: Add missing iounmap

The function of_iomap returns the result of calling ioremap, so iounmap
should be called on the result in the error handling code, as done in the
normal exit of the function.

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

)

// <smpl>
@r exists@
local idexpression x;
expression E,E1;
identifier l;
statement S;
@@

*x = of_iomap(...);
...  when != iounmap(x)
     when != if (...) { ... iounmap(x); ... }
     when != E = x
     when any
(
if (x == NULL) S
|
if (...) {
  ... when != iounmap(x)
      when != if (...) { ... iounmap(x); ... }
(
  return <+...x...+>;
|
*  return ...;
)
}
)
... when != x = E1
    when any
iounmap(x);
// </smpl>

Signed-off-by: Julia Lawall <julia@diku.dk>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>