[SCSI] bnx2fc: Fix MTU issue by using static MTU

bnx2x now uses seperate MTUs for networking and FCoE. FCoE MTU is fixed to 2500
and bnx2fc now needs to match this logic by using FCOE_MTU instead of
netdev->mtu.

Signed-off-by: Bhanu Prakash Gollapudi <bprakash@broadcom.com>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>