net: tcp2: added struct const to *net_context_state()

When compiling with CONFIG_NET_NATIVE=n and CONFIG_NET_OFFLOAD=n
then the following error is printed.

```
In file included from zephyr/subsys/net/ip/ net_if.c:23:0:
zephyr/subsys/net/ip/net_private.h: In function 'net_context_state':
zephyr/subsys/net/ip/net_private.h:58:27:
error: type of 'context' defaults to 'int' [-Werror=implicit-int]
 static inline const char *net_context_state(context)
```

This add `struct net_context *` as type for context.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>