phy: fixed_phy: Set supported speed in phydev

Set the supported field of the phydev to indicate the speed features
of the phy. If the phy is never attached to a netdev, but used in an
adjust_link() function, the speed will be incorrectly evaluated to
10/half rather than the correct speed/duplex.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>