windfarm: decrement client count when unregistering

wf_unregister_client() increments the client count when a client
unregisters. That is obviously incorrect. Decrement that client count
instead.

Fixes: 75722d39 ("[PATCH] ppc64: Thermal control for SMU based machines")

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>