add string desc helper

src/common/tusb_types.h

@@ -416,9 +416,14 @@ static inline uint8_t descriptor_len(uint8_t const p_desc[])
   return p_desc[DESC_OFFSET_LEN];
 }
 
+// Length of the string descriptors in bytes with slen characters
+#define TUD_DESC_STRLEN(_slen)      (2*(_slen) + 2)
+
+// Header of string descriptors with len + string type
+#define TUD_DESC_STR_HEADER(_slen)  ( (uint16_t) ( (TUSB_DESC_STRING << 8 ) | TUD_DESC_STRLEN(_slen)) )
 
 // Convert comma-separated string to descriptor unicode format
-#define TUD_DESC_STRCONV( ... )  (const uint16_t[]) { (TUSB_DESC_STRING << 8 ) | (2*VA_ARGS_NUM_(__VA_ARGS__) + 2), __VA_ARGS__ }
+#define TUD_DESC_STRCONV( ... )     (const uint16_t[]) { TUD_DESC_STR_HEADER(VA_ARGS_NUM_(__VA_ARGS__)), __VA_ARGS__ }
 
 #ifdef __cplusplus
  }