net: gem: Disable PCS autonegotiation in case of fixed-link

Disable PCS autonegotiation if fixed-link node is present in device tree.
This way systems with multiple GEM instances with a combination of
SGMII-fixed and SGMII-PHY will work.

Reported-by: Goran Marinkovic <goran.marinkovic@psi.ch>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Ashok Reddy Soma <ashok.reddy.soma@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>