mac80211_hwsim: use signed net namespace ID

The API expects a pointer to a signed int so we should not use an
unsigned int for it.
Signed-off-by: NJohannes Berg <johannes.berg@intel.com>

drivers/net/wireless/mac80211_hwsim.c

@@ -250,7 +250,7 @@ static inline void hwsim_clear_chanctx_magic(struct ieee80211_chanctx_conf *c)
 	cp->magic = 0;
 }
 
-static unsigned int hwsim_net_id;
+static int hwsim_net_id;
 
 static int hwsim_netgroup;