net: zynq_gem: Add support for fixed-link phy

Based on dt-specs fixed-link doesn't require phy-handle to be used.
Fix driver to only read phy related setting when phy-handle is found.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>