net: zynq_gem: Return error incase of invalid phy address

Return error from probe in case of invalid phy address.
This fixes the issue of uboot crash if phy is not detected.

Signed-off-by: Siva Durga Prasad Paladugu <sivadur@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>