amd-xgbe-phy: Use phydev advertising field vs supported

With ethtool being able to control what is advertised, the advertising
field is what should be used for priming the auto-negotiation registers
and for various other checks, instead of the supported field.

Also, move the initial setting of the supported and advertising fields
into the probe function so that they are not reset each time the device
is brought up, thus allowing the user to set as desired before bringing
the device up.

Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: David S. Miller <davem@davemloft.net>