hw/net/imx_fec: return 0xffff when accessing non-existing PHY

hw/net/imx_fec: return 0xffff when accessing non-existing PHY

If a PHY does not exist, attempts to read from it should return 0xffff.
Otherwise the Linux kernel will believe that a PHY is there and select
the non-existing PHY. This in turn will result in network errors later
on since the real PHY is not selected or configured.

Since reading from or writing to a non-existing PHY is not an emulation
error, replace guest error messages with traces.

Fixes: 461c51ad4275 ("Add a phy-num property to the i.MX FEC emulator")
Cc: Jean-Christophe Dubois <jcd@tribudubois.net>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Jason Wang <jasowang@redhat.com>