net: phy: don't change modes we don't care about in genphy_c45_read_lpa

Because 1000BaseT isn't covered by Clause 45, the 1000BaseT flags in
phydev->lp_advertising may have been set based on vendor registers
already. genphy_c45_read_lpa() would clear these flags as of today.
Therefore switch to mii_lpa_mod_linkmode_lpa_t.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>