staging: rtl8723bs: reduce stack usage of cfg80211_rtw_scan

The build of xtensa allmodconfig gives warning of:
In function 'cfg80211_rtw_scan':
warning: the frame size of 1040 bytes is larger than 1024 bytes

Allocate memory for ssid dynamically to reduce the stack usage, as an
added benifit we can remove the memset by using kzalloc while allocating
memory.
Signed-off-by: Sudip Mukherjee <sudipm.mukherjee@gmail.com>
Link: https://lore.kernel.org/r/20191018101854.31876-1-sudipm.mukherjee@gmail.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/rtl8723bs/os_dep/ioctl_cfg80211.c

@@ -1513,7 +1513,7 @@ static int cfg80211_rtw_scan(struct wiphy *wiphy
 	int i;
 	u8 _status = false;
 	int ret = 0;
-	struct ndis_802_11_ssid ssid[RTW_SSID_SCAN_AMOUNT];
+	struct ndis_802_11_ssid *ssid = NULL;
 	struct rtw_ieee80211_channel ch[RTW_CHANNEL_SCAN_AMOUNT];
 	u8 survey_times =3;
 	u8 survey_times_for_one_ch =6;
@@ -1604,7 +1604,13 @@ static int cfg80211_rtw_scan(struct wiphy *wiphy
 		goto check_need_indicate_scan_done;
 	}
 
-	memset(ssid, 0, sizeof(struct ndis_802_11_ssid)*RTW_SSID_SCAN_AMOUNT);
+	ssid = kzalloc(RTW_SSID_SCAN_AMOUNT * sizeof(struct ndis_802_11_ssid),
+		       GFP_KERNEL);
+	if (!ssid) {
+		ret = -ENOMEM;
+		goto check_need_indicate_scan_done;
+	}
+
 	/* parsing request ssids, n_ssids */
 	for (i = 0; i < request->n_ssids && i < RTW_SSID_SCAN_AMOUNT; i++) {
 		#ifdef DEBUG_CFG80211
@@ -1648,6 +1654,7 @@ static int cfg80211_rtw_scan(struct wiphy *wiphy
 	}
 
 check_need_indicate_scan_done:
+	kfree(ssid);
 	if (need_indicate_scan_done)
 	{
 		rtw_cfg80211_surveydone_event_callback(padapter);