iTCO_wdt: fix typo when setting TCO_EN bit

iTCO_wdt: fix typo when setting TCO_EN bit

The code used '&= 0x00002000' when it tried to set the TCO_EN bit, which
obviously didn't set that bit at all, but instead just reset all the
other bits in the SMI_EN register.

This bug seemingly caused various random behavior, with Frans Pop
reporting that X.org just silently hung at startup and Rafael Wysocki
reports the fan spinning with full speed.

See
	http://lkml.org/lkml/2008/12/3/178
	http://bugzilla.kernel.org/show_bug.cgi?id=12162

The problem seems to have been triggered by "[WATCHDOG] iTCO_wdt :
problem with rebooting on new ICH9 based motherboards" (commit
7cd5b08be3c489df11b559fef210b81133764ad4), but the bogus code existed
before that too (in the "supermicro_old_pre_stop()" function), it just
apparently never showed up due to different logic.

In that commit the broken code got moved around and now gets executed
much more.

Reported-by: Rafael J. Wysocki <rjw@sisk.pl>
Tested-by: Frans Pop <elendil@planet.nl>
Cc: Wim Van Sebroeck <wim@iguana.be>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>