i40e: fold prefix strings directly into stat names

We always prefix these stats with a fixed string, so just fold this
prefix into the stat string definition. This preparatory work will make
it easier to implement a helper function to copy stats and strings into
the supplied buffers in a future patch.

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>