net: ppp_generic - use DEFINE_IDR for static initialization

We could use DEFINE_IDR for statically allocated idr
that allow us to save a few lines of code.

And spell fix.

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>