sfc: Add support for reading packet length from prefix

Define a flag for struct efx_rx_buffer and efx_rx_packet() that
indicates packet length must be read from the prefix.  If this
is set, read the length in __efx_rx_packet() (when the prefix
should have arrived in cache).

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>

diff --cc drivers/net/ethernet/sfc/net_driver.h
Simple merge

diff --cc drivers/net/ethernet/sfc/rx.c
Simple merge