ethtool: Clean up definitions of rule location arrays in RX NFC

Correct the description of ethtool_rxnfc::rule_locs; it is an array
of currently used locations, not all possible valid locations.

Add note that drivers must not use ethtool_rxnfc::rule_locs.

The rule_locs argument to ethtool_ops::get_rxnfc is either NULL or a
pointer to an array of u32, so change the parameter type accordingly.

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