[SCSI] fcoe, fnic, libfc: increased CDB size to 16 bytes for fcoe.

No reason to restrict CDB size to 12 bytes in fcoe, so
increased to 16 so that 16 bytes SCSI CDB doesn't fail.

Uses common define to set max_cmd_len for fcoe and fnic,
fnic is already setting max_cmd_len to 16.

sg_readcap -l fails without this fix.

Signed-off-by: Vasu Dev <vasu.dev@intel.com>
Signed-off-by: Robert Love <robert.w.love@intel.com>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>