modem: quectel-bg9x: use NET_SOCKETS_OFFLOAD_PRIORITY

Use NET_SOCKETS_OFFLOAD_PRIORITY instead of
NET_SOCKETS_OFFLOAD_PRIORITY, so that by default offloaded sockets will
be used instead of native sockets.

Signed-off-by: Marcin Niestroj <m.niestroj@emb.dev>