i40e: use cpumask_copy instead of direct assignment

According to the header file cpumask.h, we shouldn't be directly copying
a cpumask_t, since its a bitmap and might not be copied correctly. Lets
use the provided cpumask_copy() function instead.

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>