bnx2x: MTU for FCoE L2 ring

Always configure an FCoE L2 ring with a mini-jumbo MTU size (2500).
To do that we had to move the rx_buf_size parameter from per
function level to a per ring level.

Signed-off-by: Vladislav Zolotarov <vladz@broadcom.com>
Signed-off-by: Eilon Greenstein <eilong@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>