fix(ble): write new mac if files does not exist

Fixes a bug introduced with !370 (merged)

epicardium/ble/ble.c

@@ -96,7 +96,7 @@ static void setAddress(void)
 
 	int result = fs_read_text_file("mac.txt", buf, sizeof(buf));
 
-	if (result == -1) {
+	if (result < 0) {
 		APP_TRACE_INFO0("mac.txt not found, generating random MAC");
 		epic_trng_read(bdAddr + 3, 3);
 		sprintf(buf,