rtl8195am - fix wifi_util.c warnings

Fix the following warnings:

[Warning] wifi_util.c@791,20: comparison between signed and unsigned integer expressions [-Wsign-compare]
[Warning] wifi_util.c@835,21: comparison between signed and unsigned integer expressions [-Wsign-compare]

Signed-off-by: Tony Wu <[email protected]>

Author: tung7970

targets/TARGET_Realtek/TARGET_AMEBA/sdk/common/api/wifi/wifi_util.c

@@ -784,7 +784,8 @@ int wext_get_scan(const char *ifname, char *buf, __u16 buf_len)
 int wext_private_command_with_retval(const char *ifname, char *cmd, char *ret_buf, int ret_len)
 {
     struct iwreq iwr;
-    int ret = 0, buf_size;
+    int ret = 0;
+    size_t buf_size;
     char *buf;
 
     buf_size = 128;
@@ -817,7 +818,8 @@ int wext_private_command_with_retval(const char *ifname, char *cmd, char *ret_bu
 int wext_private_command(const char *ifname, char *cmd, int show_msg)
 {
     struct iwreq iwr;
-    int ret = 0, buf_size;
+    int ret = 0;
+    size_t buf_size;
     char *buf;
 
     u8 cmdname[17] = {0}; // IFNAMSIZ+1