sfc: Fix reading of inserted hash

The hash appears immediately before the packet data, not at the
beginning of the buffer. This means we can easily use negative offsets
from the start of packet data, so adjust the data and length at the
top of __efx_rx_packet() instead of wherever we consume the hash.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>