ixgbe: clear RNBC only for 82598

RNBC (0x03FC0) is only for 82598 and has different meaning
on newer HW. Make sure to only clear it for 82598.

Signed-off-by: Emil Tantilov <emil.s.tantilov@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>