net: gtp: Move net_device assigned in setup

Assign netdev to gtp->dev at setup time, so, we can get rid of
gtp_dev_init() completely.

Signed-off-by: Breno Leitao <leitao@debian.org>
Acked-by: Pablo Neira Ayuso <pablo@netfilter.org>
Link: https://lore.kernel.org/r/20240305121524.2254533-3-leitao@debian.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>