ixgbe: Fix NAPI enable/disable path when using DCB

This change allows DCB mode to change the number of queues, and presumably
the number of NAPI instances, safely.

Signed-off-by: Peter P Waskiewicz Jr <peter.p.waskiewicz.jr@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>