net: Define NETDEV_FCOE_WWNN, NETDEV_FCOE_WWPN only when CONFIG_LIBFCOE is enabled

bnx2fc driver calls netdev->netdev_ops->ndo_fcoe_get_wwn() and it may not
be defined with the current Kconfig dependencies.  ndo_fcoe_get_wwn is
dependent on CONFIG_FCOE, but bnx2fc does not select CONFIG_FCOE, as it does
not depend on fcoe driver. Since both fcoe and bnx2fc drivers select
CONFIG_LIBFCOE, define NETDEV_FCOE_WWNN and NETDEV_FCOE_WWPN when
CONFIG_LIBFCOE is defined.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Reported-by: Randy Dunlap <rdunlap@xenotime.net>
Cc: Yi Zou <yi.zou@intel.com>
Signed-off-by: Bhanu Prakash Gollapudi <bprakash@broadcom.com>
Signed-off-by: Michael Chan <mchan@broadcom.com>
Reviewed-by: Yi Zou <yi.zou@intel.com>
Acked-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: David S. Miller <davem@davemloft.net>