skge: race with workq and RTNL

If a workqueue function that needs RTNL is running when skge_down
is called then a deadlock is possible. Fix by only clearing the timer,
and handling the flush_scheduled_work on removal. This work queue is only
ever used for the old fiber based boards.

Signed-off-by: Stephen Hemminger <shemminger@linux-foundation.org>
Signed-off-by: Jeff Garzik <jeff@garzik.org>