sfc: Specify a meaningful component for loopback RX-side and PHY tests

Our ethtool self-test result names each begin with a component name.  For
some results this is "port0", which is not very meaningful.  Change that
to "rx" or "phy" as appropriate.

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