igb[v],ixgbe: don't use flush_scheduled_work()

All three drivers use flush_scheduled_work() similarly during driver
detach.  Replace it with explicit cancels.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: e1000-devel@lists.sourceforge.net
Cc: netdev@vger.kernel.org