[POWERPC] windfarm: don't die on suspend thread signal

When the windfarm thread gets a suspend signal it will die instead of
freezing. This fixes it.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>