ixgbe: DCB, use hardware independent routines

This consolidates hardware specifics to ixgbe_dcb.c this simplifies
code that was previously branching based on hardware type.

Signed-off-by: John Fastabend <john.r.fastabend@intel.com>
Tested-by: Ross Brattain <ross.b.brattain@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>