drivers/net: remove unnecessary flush_scheduled_work() calls

janz-ican3, sh_eth, skge and vxge don't use workqueue at all and there
is no reason to flush the system_wq.  Drop flush_scheduled_work()
calls and references to workqueue.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Wolfgang Grandegger <wg@grandegger.com>
Cc: Stephen Hemminger <shemminger@linux-foundation.org>
Cc: Ramkrishna Vepa <ramkrishna.vepa@exar.com>
Cc: Sivakumar Subramani <sivakumar.subramani@exar.com>
Cc: Sreenivasa Honnur <sreenivasa.honnur@exar.com>
Cc: Jon Mason <jon.mason@exar.com>
Cc: netdev@vger.kernel.org