ixgbe: dcbnl reduce duplicated code and indentation

Replace duplicated code in if/else branches with single
check and ixgbe_init_interrupt_scheme().

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>