staging: vchiq: switch to wait_for_completion_killable

This fixes f27e47bc ("staging: vchiq: use completions instead of
semaphores") as it neglected the subtle down_interruptible() macro
override in vchiq_killable.h. Hence all completions should be killable
instead of interruptible.

Fixes: f27e47bc

 ("staging: vchiq: use completions instead of semaphores")
Reported-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>