net: ne2000: fix bounds check in ioport operations

While doing ioport r/w operations, ne2000 device emulation suffers
from OOB r/w errors. Update respective array bounds check to avoid
OOB access.

Reported-by: Ling Liu <liuling-it@360.cn>
Cc: qemu-stable@nongnu.org
Signed-off-by: Prasad J Pandit <pjp@fedoraproject.org>
Signed-off-by: Jason Wang <jasowang@redhat.com>
