wan/hdlc_x25: make lapb params configurable

This enables you to configure mode (DTE/DCE), Modulo, Window, T1, T2, N2 via
sethdlc (which needs to be patched as well).

Signed-off-by: Martin Schiller <ms@dev.tdt.de>
Signed-off-by: David S. Miller <davem@davemloft.net>