usb hub: use flush_work instead of flush_work_sync

flush_work_sync and flush_work are now the same and flush_work_sync
has been deprecated. This fixes the following warning:

drivers/usb/core/hub.c: In function hub_quiesce:
drivers/usb/core/hub.c:1216:3: warning: flush_work_sync is deprecated (declared at include/linux/workqueue.h:448) [-Wdeprecated-declarations]

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Octavian Purdila <octavian.purdila@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>