xen-pcifront: don't use flush_scheduled_work()

flush_scheduled_work() is scheduled for deprecation.  Cancel ->op_work
directly instead.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Ryan Wilson <hap9@epoch.ncsc.mil>
Cc: Jan Beulich <JBeulich@novell.com>
Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>