ifpps: correct calculation of line rate percentage

The line rate is given in Mbit rather than MiB (in which {rx,tx}_bytes
were converted). Correct the calculation by converting {rx,tx}_bytes to
Mbit:

  (n bytes * 8 bits per byte) / 1000^2 = (n bytes) / 125000

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>