watchdog: intel_scu_watchdog: Remove unused variable

Variable "hw_pre_value" is not used anywhere in the
function intel_scu_set_heartbeat().
This patch removes this variable.

Signed-off-by: Alexander Shiyan <shc_work@mail.ru>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>