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: NBen Hutchings <bhutchings@solarflare.com>