ixgbe: add new bridge mode support function.

This patch gathers together all the logic needed to configure bridge
modes.  Currently that it is rather simple but this is really laying
the ground work for future X550 feature enhancement.

Signed-off-by: Don Skidmore <donald.c.skidmore@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>