xen: don't use flush_scheduled_work()

flush_scheduled_work() is deprecated and scheduled to be removed.
Directly flush info->work instead.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>