[SCSI] libfcoe: Move FCOE_MTU definition from fcoe.h to libfcoe.h

both fcoe and bnx2fc drivers can access the common definition of
FCOE_MTU.

Signed-off-by: Bhanu Prakash Gollapudi <bprakash@broadcom.com>
Signed-off-by: Robert Love <robert.w.love@intel.com>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>