be2net: fix bad queue traversal in be_rx_q_clean()

Using "for(tail != head)" to traverse a queue from tail to head
fails in the case of a fully filled queue. Use "for(used != 0)" instead.

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