ks8842: Support 100Mbps when accessed via timberdale

This patch removes the code which disables 100Mbps advertising when
the ks8842 is accessed via timberdale.

At higher speed it's good to be nice to the internal state machine
of timberdale by acking interrupts. That is done by a write to the
interrupt ack register (IAR).

Signed-off-by: Richard Röjfors <richard.rojfors@pelagicore.com>
Signed-off-by: David S. Miller <davem@davemloft.net>