be2net: Handle out of buffer completions for lancer

If Lancer chip does not have posted RX buffers, it posts an RX completion entry
with the same frag_index as the last valid completion. The Error bit is also
set. In BE, a flush completion is indicated with a zero value for num_rcvd in
the completion.
Such completions don't carry any data and are not processed.
This patch refactors code to handle both cases with the same code.

Signed-off-by: Padmanabh Ratnakar <padmanabh.ratnakar@emulex.com>
Signed-off-by: Sathya Perla <sathya.perla@emulex.com>
Signed-off-by: David S. Miller <davem@davemloft.net>