i40e: simplify member variable accesses

This commit replaces usage of vsi->back in i40e_print_link_message()
(which is actually a PF pointer) with temp variable.

Signed-off-by: Sudheer Mogilappagari <sudheer.mogilappagari@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>