net: ppp: Fix for coverity CID 203514

Looks like the logic to count rejection is missing. Removing count_rej
variable, and set a comment about initializing the code to the right
value once this logic will be in.

Coverity-CID: 203514
Fixes #18398

Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>