ixgbe: Allow Priority Flow Control settings to survive a device reset

When changing DCB parameters, ixgbe needs to have the MAC reset.  The way
the flow control code is setup today, PFC will be disabled on a reset.
This patch adds a new flow control type for PFC, and then has the netlink
layer take care of toggling which type of flow control to enable.

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>