ixgbe: set the correct DCB bit for pg tx settings

Set the correct bit BIT_PG_TX when tx PG settings are set.

Signed-off-by: John Fastabend <john.r.fastabend@intel.com>
Acked-by: PJ Waskiewicz <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>