kernel: vrx518_tc: fix ADSL/ATM operation

kernel: vrx518_tc: fix ADSL/ATM operation

ATM TC layer have some issues which effectively prevent VRX518 from
being used as ADSL modem. Specifically, there one crash during the ATM
layer configuration and wrong PVC ID selection on packet receiving what
breaks RX path. Fix both of the issues. Make subif iface registration
optional to prevent the crash (see more details in the new patch) and
update the hardcoded PVC ID to match the first allocated channel.

Run tested with FRITZ!Box 7530.

Fixes: 474bbe23b7 ("kernel: add Intel/Lantiq VRX518 TC driver")
Reported-and-tested-by: nebibigon93@yandex.ru
Signed-off-by: Sergey Ryazanov <ryazanov.s.a@gmail.com>
Link: https://patchwork.ozlabs.org/project/openwrt/patch/20250122222654.21833-3-ryazanov.s.a@gmail.com/
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
(cherry picked from commit 470335450e67002366fcbcd7334b15bdf008e44d)