ehea: don't use flush_scheduled_work()

Directly cancel port->reset_task from ehea_shutdown_single_port()
instead.  As this cancels the work for each port on driver detach,
flushing system_wq from ehea_remove() or ehea_module_exit() is no
longer necessary.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Breno Leitao <leitao@linux.vnet.ibm.com>
Cc: netdev@vger.kernel.org