net: zynq_gem: Do not return -ENOSYS on success

net: zynq_gem: Do not return -ENOSYS on success

The .read_rom_hwaddr net_ops hook does not check the return value, which
is why it was never caught that we are currently returning 0 if the
read_rom_hwaddr function return -ENOSYS and -ENOSYS otherwise.

In this case we can simplify this by just returning the result of the
function.

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