net: ethtool: disambiguate XCVR_* meaning

Add a comment which explains the real meaning of XCVR_INTERNAL (PHY and
Ethernet MAC in the same package/die) and XCVR_EXTERNAL (PHY and
Ethernet MAC in a different package/die). Most if not all of the drivers
setting their transceiver type already do it the way the comment
describes it.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>