ixgbe: Fix DCB netlink layer for 82599 to enable Priority Flow Control

The priority flow control settings from the netlink layer aren't taking
effect in the base driver.  The boolean pfc_mode_enable in the dcb_config
struct isn't being set, so the hardware configuration code is never
reached.

Signed-off-by: Peter P Waskiewicz Jr <peter.p.waskiewicz.jr@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>