hw/net/e1000: fix integer endianness

It's detected by coverity.In is_vlan_packet s->mac_reg[VET] is
unsigned int but is dereferenced as a narrower unsigned short.
This may lead to unexpected results depending on machine
endianness.

Signed-off-by: Shannon Zhao <zhaoshenglong@huawei.com>
Signed-off-by: Shannon Zhao <shannon.zhao@linaro.org>
Message-id: 1426224119-8352-1-git-send-email-zhaoshenglong@huawei.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>