sfc: remove obsolete counters from struct efx_channel

The n_rx_tobe_disc and n_rx_mcast_mismatch counters are a legacy
 from farch, and are never written in EF10 or EF100 code.  Remove
 them from the struct and from ethtool -S output, saving a bit of
 memory and avoiding user confusion.

Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: Edward Cree <ecree.xilinx@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>