[WATCHDOG] iTCO_wdt : problem with rebooting on new ICH9 based motherboards

Bugzilla #9868: On Intel motherboards with the ICH9 based I/O controllers
(Like DP35DP and DG33FB) the iTCO timer counts but it doesn't reboot the
system after the counter expires.

This patch fixes this by moving the enabling & disabling of the TCO_EN bit
in the SMI_EN register into the start and stop code.

Signed-off-by: Wim Van Sebroeck <wim@iguana.be>