watchdog: fix hpwdt Kconfig regression in 3.0-rc

Remove Kconfig regression caused by commit
a4616153 "watchdog: hpwdt: build hpwdt as
module by default with NMI_DECODING enabled"

With the above change applied, hpwdt will be enabled unconditionally by just
entering the Watchdog subscreen in menuconfig. Since this driver is not
essential to boot any box it should remain disabled until it gets manually
enabled, just like all other drivers.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Cc: Tony Camuso <tcamuso@redhat.com>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>