sfc: Generalise packet hash lookup to support EF10 RX prefix

EF10 uses an entirely different RX prefix format from Falcon-arch.
Extend struct efx_nic_type to describe this.

[bwh: Also replace the magic numbers used for the Falcon-arch RX prefix]
Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>