net: phy: deal properly with autoneg incomplete in genphy_c45_read_lpa

The link partner advertisement registers are not guaranteed to contain
valid values if autoneg is incomplete. Therefore, if
MDIO_AN_STAT1_COMPLETE isn't set, let's clear all link partner
capability bits. This also avoids unnecessary register reads if link
is down and phylib is in polling mode.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>