net: ethernet: gianfar_ethtool: remove redundant variable last_rule_idx

Variable last_rule_idx is being assigned but is never used hence it is
redundant and can be removed.

Cleans up clang warning:
warning: variable 'last_rule_idx' set but not used [-Wunused-but-set-variable]

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Claudiu Manoil <claudiu.manoil@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>