i40e: use declared variables for pf and hw

In order to slightly simplify the code use the variables for pf and hw
that are declared in i40e_set_mac function.

Signed-off-by: Patryk Małek <patryk.malek@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>