ifpps: make '+' to '-' sort more clear

If will now look like the following:

  cpu6 +:       237357 irqs
  cpu2 |:       117538 irqs
  cpu0 |:         8419 irqs
  cpu7 |:         4523 irqs
  cpu5 |:         4312 irqs
  cpu1 -:         1432 irqs
  avg:           47606.8

This makes it a bit more precise/intuitive that we sort from max
to min.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
link: allow future ethtool speed formats to be supported

In case some in the near future NICs with 40Gbit/s will appear. Allow
ifpps to get the correct ethtool speed.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>