Rename mii_stat1000_to_linkmode_lpa_t to
mii_stat1000_mod_linkmode_lpa_t to indicate it modifies the passed
linkmode bitmap, without clearing any other bits.

Add a helper to set/clear bits in a linkmode.

Use this helper to ensure bit are clear which the stat1000 indicates
should not be set.

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