Documentation: net-sysfs: Remove duplicate PHY device documentation

Both sysfs-bus-mdio and sysfs-class-net-phydev contain the same
duplication information. There is not currently any MDIO bus specific
attribute, but there are PHY device (struct phy_device) specific
attributes. Use the more precise description from sysfs-bus-mdio and
carry that over to sysfs-class-net-phydev.

Fixes: 86f22d04 ("net: sysfs: Document PHY device sysfs attributes")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>