don't index arrays with signed types

modules/Ethernet/private_mib_bt6709.c

@@ -295,12 +295,11 @@ void lwip_privmib_init(void)
 uint32_t check_netsettings(char *addr, uint8_t len)
 {
 	uint32_t val = 0;
-	char c;
 	uint8_t i, j;
 	uint8_t cnt_dot = 0;
 	uint8_t digit = 0;
 
-	c = *addr;
+	uint8_t c = *addr;
 
 	for(j = 0; j <= len; j ++){
 		if (isdigit(c)) {