net: mii: Fix autoneg in mii_lpa_to_linkmode_lpa_t()

mii_adv_to_linkmode_adv_t() clears all bits before setting it needs to
set. This means the freshly set Autoneg gets cleared.

Change the order, and add comments about it clearing the old content
of the bitmap.

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>