cfg80211: initialize reg_rule in __freq_reg_info()

Sparse started warning on this function because we can potentially
return an uninitialized value.  The reason is that if the caller
passes a min_bw value that is higher then the last value in bws[], we
will not go into the loop and reg_rule will remain initialized.  This
cannot happen because the only caller of this function uses either 1
or 20 in min_bw, but the function will be more robust if we
pre-initialize the value.
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://lore.kernel.org/r/iwlwifi.20210204154439.6c884ea7281c.I257278d03b0c1ae0aa6631672cfa48f1a95d5996@changeidSigned-off-by: Johannes Berg <johannes.berg@intel.com>

net/wireless/reg.c

@@ -1629,7 +1629,7 @@ __freq_reg_info(struct wiphy *wiphy, u32 center_freq, u32 min_bw)
 {
 	const struct ieee80211_regdomain *regd = reg_get_regdomain(wiphy);
 	static const u32 bws[] = {0, 1, 2, 4, 5, 8, 10, 16, 20};
-	const struct ieee80211_reg_rule *reg_rule;
+	const struct ieee80211_reg_rule *reg_rule = ERR_PTR(-ERANGE);
 	int i = ARRAY_SIZE(bws) - 1;
 	u32 bw;