net: phy: add qca8081 config_aneg

Reuse at803x phy driver config_aneg excepting
adding 2500M auto-negotiation.

Signed-off-by: Luo Jie <luoj@codeaurora.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>