net: l2: ppp: drop ppp_context's is_network_up

There is already a variable 'network_protos_up', which stores number of
network protocols being up. Additionally each network protocol has its
own state represented by is_ip{,v6}cp_up. Use the latter in FSM up() and
down() callbacks.

This fixes a case when both IPCP and IPv6CP protocols are going
down. When using ctx->is_network_up only one of them (the first) was
deinitialized correctly, second stayed always up (at least partially,
e.g. not calling ppp_network_down()).

Signed-off-by: Marcin Niestroj <m.niestroj@grinn-global.com>