sfc: add ability for an RXQ to grant credits on refill

EF100 hardware streams MAE counter updates to the driver over a dedicated
 RX queue; however, the MCPU is not able to detect when RX buffers have
 been posted to the ring.  Thus, the driver must call
 MC_CMD_MAE_COUNTERS_STREAM_GIVE_CREDITS; this patch adds the
 infrastructure to support that to the core RXQ handling code.

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