amd-xgbe: handle corner-case during sfp hotplug

Force the mode change for SFI in Fixed PHY configurations. Fixed PHY
configurations needs PLL to be enabled while doing mode set. When the
SFP module isn't connected during boot, driver assumes AN is ON and
attempts auto-negotiation. However, if the connected SFP comes up in
Fixed PHY configuration the link will not come up as PLL isn't enabled
while the initial mode set command is issued. So, force the mode change
for SFI in Fixed PHY configuration to fix link issues.

Fixes: e57f7a3f ("amd-xgbe: Prepare for working with more than one type of phy")
Acked-by: Shyam Sundar S K <Shyam-sundar.S-k@amd.com>
Signed-off-by: Raju Rangoju <Raju.Rangoju@amd.com>
Reviewed-by: Wojciech Drewek <wojciech.drewek@intel.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>