mn10300: Fix crash just after starting userspace on !CONFIG_PREEMPT

The mn10300 kernel crashes just after starting userspace programs, if
CONFIG_PREEMPT is disabled:

  Freeing unused kernel memory: 96K (90286000 - 9029e000)
  MISALIGN: 97c33ff9: unsupported instruction f
  MISALIGN: 97c33ff9: unsupported instruction f
  MISALIGN: 97c33ff9: unsupported instruction f
  :

This fixes the problem that was introduced by commit d17fc238
("MN10300: Enable IRQs more in system call exit work path").

Signed-off-by: Akira Takeuchi <takeuchi.akr@jp.panasonic.com>
Signed-off-by: Kiyoshi Owada <owada.kiyoshi@jp.panasonic.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>