x86: nmi_watchdog - use nmi_watchdog variable for printing

Since it is possible NMI_ definitions could be changed
one day we better print out real nmi_watchdog value instead
of constant string.

Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>
Cc: macro@linux-mips.org
Signed-off-by: Ingo Molnar <mingo@elte.hu>