ethtool: Explicitly state that RX NFC rule locations are priorities

The location of an RX flow classification rule is needed to identify
it for retrieval, replacement or deletion.  However it also defines
the priority of the rule in the case that a flow is matched by
multiple rules.  This is what I intended to imply by referring to the
use of a TCAM, commonly used to implement that behaviour.

However there are other ways this can be done, and it is better to
specify this explicitly.  Further, I want to add the option for
automatic selection of rule locations.

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