net: phy: at803x: finish the phy id checking simplification

The blamed commit was probably not tested on net-next, since it did not
refactor the extra phy id check introduced in commit b856150c ("net:
phy: at803x: mask 1000 Base-X link mode").

Fixes: 8887ca54 ("net: phy: at803x: simplify custom phy id matching")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Acked-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>