ehea: fix race condition

When ehea_stop is called the function
cancel_work_sync(&port->reset_task) is used to ensure
that the reset task is not running anymore. We need an
additional flag to ensure that it can not be scheduled
after this call again for a certain time.

Signed-off-by: Jan-Bernd Themann <themann@de.ibm.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>