[POWERPC] windfarm: Fix windfarm thread freezer interaction

[POWERPC] windfarm: Fix windfarm thread freezer interaction

When I fixed the windfarm freezer interaction first in commit
8986909d2cc93ee19e36ce7f4fbc9c3f0666dc50, an earlier patch than the one
I came up with after comments was committed. This has come back to haunt
us now because commit 906003e6e979fec198f7adf44d14cef56eb40175 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 8986909d2cc93ee19e36ce7f4fbc9c3f0666dc50 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>