[WATCHDOG] hpwdt: Fix NMI handling.

I need to just return in case it's not my NMI so someone else can take a look
at it (and reset die_nmi_called to 0 in case I actually do get one that's mine
to handle).

Signed-off-by: Thomas Mingarelli <thomas.mingarelli@hp.com>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>