net: mii: Add mii_lpa_mod_linkmode_lpa_t

Add a _mod_ variant of mii_lpa_to_linkmode_lpa_t. Use this to fix the
genphy_read_status() where the 1G link partner features are getting
lost.

Fixes: c0ec3c27 ("net: phy: Convert u32 phydev->lp_advertising to linkmode")
Reported-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>