nmi_watchdog: Properly configure for software events

Paul Mackerras brought up a good point that when fallbacking to
software events, I may have been lucky in my configuration.

Modified the code to explicit provide a new configuration for
software events.

Suggested-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Don Zickus <dzickus@redhat.com>
Cc: gorcunov@gmail.com
Cc: aris@redhat.com
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
LKML-Reference: <1266357745-26671-1-git-send-email-dzickus@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>