ixgbe: make silicon specific functions generic

The new MAC type X540 shares much of the same functionality of
some silicon specific functions.  To reduce duplicate code,
made these functions generic.

Signed-off-by: Don Skidmore <donald.c.skidmore@intel.com>
Tested-by: Stephen Ko <stephen.s.ko@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>