Make hifn_795x::hifn_remove() call cancel_delayed_work_sync() instead
of calling cancel_delayed_work() followed by flush_scheduled_work().

This is to prepare for the deprecation and removal of
flush_scheduled_work().
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>