hw/net/smc91c111: Use qemu_log_mask(UNIMP) instead of fprintf

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-id: 20180624040609.17572-15-f4bug@amsat.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>