amd-xgbe: Fix initial mode when auto-negotiation is disabled

When the ethtool command is used to set the speed of the device while
the device is down, the check to set the initial mode may fail when
the device is brought up, causing failure to bring the device up.

Update the code to set the initial mode based on the desired speed if
auto-negotiation is disabled.

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