sfc: add ability for extra channels to receive raw RX buffers

The TC extra channel will need its own special RX handling, which must
 operate before any code that expects the RX buffer to contain a network
 packet; buffers on this RX queue contain MAE counter packets in a
 special format that does not resemble an Ethernet frame, and many fields
 of the RX packet prefix are not populated.
The USER_MARK field, however, is populated with the generation count from
 the counter subsystem, which needs to be passed on to the RX handler.

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