watchdog: hpwdt: condition early return of NMI handler on iLO5

Modify prior change to not claim an NMI unless originated
from iLO to apply only to iLO5 and later going forward.
This restores hpwdt traditional behavior of calling panic
if the NMI is NMI_IO_CHECK, NMI_SERR, or NMI_UNKNOWN for
legacy hardware.

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