powerpc/windfarm: Fix overtemperature clearing

With pm81/pm91/pm121, when the overtemperature state is entered, and
when it remains on after skipped ticks, the driver will try to leave
it too soon (immediately on the next tick). This is because the active
FAILURE_OVERTEMP state is not visible in "new_failure" variable of the
current tick. Furthermore, the driver will keep trying to clear condition
in subsequent ticks as FAILURE_OVERTEMP remains set in the "last_failure"
variable. These will start to trigger WARNINGS from windfarm core:

[  100.082735] windfarm: Clamping CPU frequency to minimum !
[  100.108132] windfarm: Overtemp condition detected !
[  101.952908] windfarm: Overtemp condition cleared !
[...]
[  102.980388] WARNING: at drivers/macintosh/windfarm_core.c:463
[...]
[  103.982227] WARNING: at drivers/macintosh/windfarm_core.c:463
[...]
[  105.030494] WARNING: at drivers/macintosh/windfarm_core.c:463
[...]
[  105.973666] WARNING: at drivers/macintosh/windfarm_core.c:463
[...]
[  106.977913] WARNING: at drivers/macintosh/windfarm_core.c:463

Fix by adding a helper global variable. We leave the overtemp state only
after all failure bits have been cleared.

I saw this error on iMac G5 iSight (pm121). Also pm81/pm91 are fixed
based on the observation that these are almost identical/copy-pasted code.

Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>