net: eepro100: fix memory leak in device uninit

The exit dispatch of eepro100 network card device doesn't free
the 's->vmstate' field which was allocated in device realize thus
leading a host memory leak. This patch avoid this.

Signed-off-by: Li Qiang <liqiang6-s@360.cn>
Signed-off-by: Jason Wang <jasowang@redhat.com>
