i40e: adds FCoE configure option

Adds FCoE config option I40E_FCOE, so that FCoE can be enabled
as needed but otherwise have it disabled by default.

This also eliminate multiple FCoE config checks, instead now just
one config check for CONFIG_I40E_FCOE.

The I40E FCoE was added with 3.17 kernel and therefore this patch
shall be applied to stable 3.17 kernel also.

CC: <stable@vger.kernel.org>
Signed-off-by: Vasu Dev <vasu.dev@intel.com>
Tested-by: Jim Young <jamesx.m.young@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>