arch/mn10300/mm: eliminate NULL dereference

dev_name always dereferences its argument, so it should not be called if
the argument is NULL.  The function indeed later tests the argument for
being NULL.

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

)

  // <smpl>
  @@
  expression dev,E;
  @@

  *dev_name(dev)
  ... when != dev = E
  (
  *dev == NULL
  |
  *dev != NULL
  )
  // </smpl>

Signed-off-by: Julia Lawall <julia@diku.dk>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>