i40e: Fix for Tx timeouts when interface is brought up if DCB is enabled

If interface is connected to switch port configured for DCB there are
TX timeouts when bringing up interface. Problem started appearing after
adding in i40e driver code mqprio hardware offload mode. In function
i40e_vsi_configure_bw_alloc was added resetting BW rate which should
be executing when mqprio qdisc is removed but was also when there was
no mqprio qdisc added and DCB was enabled. In this patch was added
additional check for DCB flag so now when DCB is enabled the correct
DCB configs from before mqprio patch are restored.

Signed-off-by: Martyna Szapar <martyna.szapar@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>