ixgbe: add new wrapper for X550 support

For the X550 mac type we have to do additional steps around
enabling/disabling Rx.  This patch will add a layer of indirection
around these support functions to enable this.

CC: <kernel-team@fb.com>
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>