e1000: disable debug by default

Disable debug output by default, the information were not needed for
release.

Cc: Peter Maydell <peter.maydell@linaro.org>
Cc: Stefan Hajnoczi <stefanha@gmail.com>
Cc: Leonid Bloch <leonid.bloch@ravellosystems.com>
Cc: Dmitry Fleytman <dmitry.fleytman@ravellosystems.com>
Cc: qemu-stable@nongnu.org
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Jason Wang <jasowang@redhat.com>