i40e: Set XPS bit mask to zero in DCB mode

Due to DCBX configuration change if the VSI needs to use more than 1 TC;
it needs to disable the XPS maps that were set when operating in 1 TC mode.
Without disabling XPS the netdev layer will select queues based on those
settings and not use the TC queue mapping to make the queue selection.

This patch allows the driver to enable/disable the XPS based on the number
of TCs being enabled for the given VSI.

Change-ID: Idc4dec47a672d2a509f6d7fe11ed1ee65b4f0e08
Signed-off-by: Neerav Parikh <neerav.parikh@intel.com>
Tested-By: Jack Morgan <jack.morgan@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>