[rdkb][common][hal][Fix Guard Interval setting issue]

[Description]
Fix Guard Interval setting issue
1. setting didn't write into flash. So after reboot, hostapd config didn't match the config of wifi agent

[Release-log]

diff --git a/src/wifi/wifi_hal.c b/src/wifi/wifi_hal.c
index d2c0e6f..d03650f 100644
--- a/src/wifi/wifi_hal.c
+++ b/src/wifi/wifi_hal.c
@@ -74,7 +74,7 @@
 #define SOCK_PREFIX "/var/run/hostapd/wifi"
 #define VAP_STATUS_FILE "/tmp/vap-status"
 #define ESSID_FILE "/tmp/essid"
-#define GUARD_INTERVAL_FILE "/tmp/guard-interval"
+#define GUARD_INTERVAL_FILE "/nvram/guard-interval"
 #define CHANNEL_STATS_FILE "/tmp/channel_stats"
 #define DFS_ENABLE_FILE "/nvram/dfs_enable.txt"
 #define VLAN_FILE "/nvram/hostapd.vlan"