net: phy: don't use 10BaseT/half as default in genphy_read_status

If link partner and we can't agree on any mode, then it doesn't make
sense to pretend we would have agreed on 10/half. Therefore set a
proper default.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>