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>

include/uapi/linux/ethtool.h

@@ -993,8 +993,8 @@ enum ethtool_sfeatures_retval_bits {
 #define PORT_OTHER		0xff
 
 /* Which transceiver to use. */
-#define XCVR_INTERNAL		0x00
-#define XCVR_EXTERNAL		0x01
+#define XCVR_INTERNAL		0x00 /* PHY and MAC are in the same package */
+#define XCVR_EXTERNAL		0x01 /* PHY and MAC are in different packages */
 #define XCVR_DUMMY1		0x02
 #define XCVR_DUMMY2		0x03
 #define XCVR_DUMMY3		0x04