watchdog: hpwdt: Claim NMI from iLO

The hwpdt driver is overloaded for handling both the iLO
watchdog and the explicit "Generate NMI to System" virutal
button.  Hence NMI handler needs to claim NMI resulting
from the virutal button.

Claim if iLO generated accommodating firmware that might
set wrong bit.

Signed-off-by: Jerry Hoemann <jerry.hoemann@hpe.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>