sfc: add start and stop methods to channels

The TC extra channel needs to do extra work in efx_{start,stop}_channels()
 to start/stop MAE counter streaming from the hardware.  Add callbacks for
 it to implement.

Signed-off-by: Edward Cree <ecree.xilinx@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>