i40e: simplify pointer dereferences

Now that i40e_vsi_config_tc() has the pf and hw variable defined, use
them, instead of dereferencing vsi->back. Much easier to read.

Signed-off-by: Mitch Williams <mitch.a.williams@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>