watchdog: hpwdt: Unregister NMI events on exit.

This patch is to unregister for NMI events upon exit. Also we are now
making the default setting for allow_kdump enabled.

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