bnx2x: Filter packets on FCoE rings

Whenever bnx2x fails to transmit a packet due to a full Tx ring, if the
ring size is zero (indicating an FCoE ring) driver filters the packet out
and gracefully continues.
Driver also gathers statistics on such filtered packets.

Signed-off-by: Dmitry Kravkov <dmitry@broadcom.com>
Signed-off-by: Yuval Mintz <yuvalmin@broadcom.com>
Signed-off-by: Eilon Greenstein <eilong@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>