ixgbe: Properly disable DCB arbiters prior to applying changes

When disabling the Rx and Tx data arbiters prior to configuration changes,
the arbiters were not being shut down properly.  This can create a race
in the DCB hardware blocks, and potentially hang the arbiters.  Also, the
Tx descriptor arbiter shouldn't be disabled when applying configuration
changes; disabling this arbiter can cause a Tx hang.

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>