gianfar: Clean up implementation of RX network flow classification

This code was cribbed from niu, so gfar_set_hash_opts() begins by
converting the ethtool flow class code into a class code for Sun
Neptune hardware, then does the same thing again for the hardware it's
really dealing with.  It may also return -1 (-EPERM) for some
unhandled ethtool flow class codes.

Remove the useless code and definitions, and fix the error code.

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