ethernet/sis: Replace synchronize_sched() with synchronize_rcu()

Now that synchronize_rcu() waits for preempt-disable regions of code
as well as RCU read-side critical sections, synchronize_sched() can be
replaced by synchronize_rcu().  This commit therefore makes this change.

Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
Cc: Francois Romieu <romieu@fr.zoreil.com>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: <netdev@vger.kernel.org>