i40e: remove workaround for resetting XPS

Since commit 3ffa037d ("i40e: Set XPS bit mask to zero in DCB mode")
we've tried to reset the XPS settings by building a custom
empty CPU mask.

This workaround is not necessary because we're not really removing the
XPS setting, but simply setting it so that no CPU is valid.

Second, we shorten the code further by using zalloc_cpumask_var instead
of a separate call to bitmap_zero().

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>