sfc: Prepare for RX scatter on EF10

RX DMA scatter is always enabled on EF10.  Adjust the common RX
completion handling to allow for this.

RX completion events on EF10 include the length used from a single
descriptor, not the cumulative length used.  Add a field to struct
efx_rx_queue to hold the cumulative length.

[bwh: Also fix a related comment]
Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>