net: PPPoPNS: Remove length argument from data_ready

The argument was removed by commit 676d2369 ("net: Fix use after free by
removing length arg from sk_data_ready callbacks") and it's presence causes
warnings like:

drivers/net/ppp/pppopns.c:296:27: warning: assignment from incompatible pointer type
  po->proto.pns.data_ready = sk_raw->sk_data_ready;

Signed-off-by: Jon Medhurst <tixy@linaro.org>
Signed-off-by: Amit Pundir <amit.pundir@linaro.org>