hw/net/stellaris_enet: Use qemu_log_mask(GUEST_ERROR) instead of hw_error

hw_error() finally calls abort(), but there is no need to abort here.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20180624040609.17572-13-f4bug@amsat.org
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>