[WATCHDOG] iTCO_wdt: fix SMI_EN regression 2

bugzilla: #12363
commit 7cd5b08b

 added a second regression:
some Dell's and Compaq's lockup on boot. So we revert most of the code.
The ICH9 reboot issue remains in place and will need some more fixing... :-(

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