be2net: fix rx-path to ignore a flush completion

The flush compl (compl with numfrags == 0; no data) is rcvd
from hw to indicate completion of RXQ destory operation. Fix
the RX path to not process it as RX data.

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