be2net: Clear pending tx completions before destroying tx queue

Clear pending tx completions in be_close() (ndo.stop method) so that after
unregister_netdev() calls be_close(), all tx skbs are freed and there are
no more tx completions when txq is destroyed.

Signed-off-by: Sathya Perla <sathyap@serverengines.com>
Signed-off-by: David S. Miller <davem@davemloft.net>