net: PPPoPNS: use updated data_ready API definition

data_ready(struct sock *) no longer takes a length argument.
Remove length argument in the implementation of this function
in PPPoPNS. It was never used anyway.

Change-Id: I457cedd375a490dd85e60f172b4122b4c7ba36f0
Signed-off-by: Matt Wagantall <mattw@codeaurora.org>