net: phy: initialise phydev speed and duplex sanely

When a phydev is created, the speed and duplex are set to zero and
-1 respectively, rather than using the predefined SPEED_UNKNOWN and
DUPLEX_UNKNOWN constants.

There is a window at initialisation time where we may report link
down using the 0/-1 values.  Tidy this up and use the predefined
constants, so debug doesn't complain with:

"Unsupported (update phy-core.c)/Unsupported (update phy-core.c)"

when the speed and duplex settings are printed.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>