[POWERPC] windfarm: Fix windfarm thread freezer interaction

[POWERPC] windfarm: Fix windfarm thread freezer interaction

When I fixed the windfarm freezer interaction first in commit
1ed2ddf380e19dafeec2150ca709ef7f4a67cd21, an earlier patch than the one
I came up with after comments was committed. This has come back to haunt
us now because commit d5d8c5976d6adeddb8208c240460411e2198b393 changed
the freezer to no long send signals. Fix it by removing the windfarm
thread's signal logic and restoring the original try_to_freeze().

We could simply revert 1ed2ddf380e19dafeec2150ca709ef7f4a67cd21 now
but I feel that the assertion that no signal is delivered to the
windfarm thread needs not be there.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Acked-by: Rafael J. Wysocki <rjw@sisk.pl>
Signed-off-by: Paul Mackerras <paulus@samba.org>