midi2: align descriptors with USB-MIDI 2.0 spec

Brings the MIDI 2.0 device driver into full conformance with USB
Device Class Definition for MIDI Devices v2.0 (USB-IF, May 2020).

- Alt 1 MS Interface Header wTotalLength now reports 0x0007 per
  Table 5-2 ("set to match bLength"), replacing the prior 0x0011
  carried over from USB-MIDI 1.0 conventions.
- GET_DESCRIPTOR class request now validates bmRequestType direction,
  type and recipient plus wIndex and wValue high byte per Section 6.
- iBlockItem in the default Group Terminal Block is driven by
  CFG_TUD_MIDI2_BLOCK_STRIDX so applications can attach a UI string
  descriptor to the block per Table 5-6.
- UMP word byte order assumption (little-endian host per Section
  3.2.2) is documented inline so future big-endian ports know where
  to wrap access with tu_htole32 / tu_le32toh.

Validated on RP2040 and ESP32-P4 under Linux kernel 6.17: lsusb -v
reports wTotalLength = 0x0007 on Alt 1 MS Header (raw bytes
07 24 01 00 02 07 00). amidi -l enumerates Group Terminals exposed
via the class-specific GET_DESCRIPTOR response.

1 files changed, 6 insertions, 0 deletions

diff --git a/src/tusb_option.h b/src/tusb_option.h
index 4483c2200..2614110fc 100644
--- a/src/tusb_option.h
+++ b/src/tusb_option.h
@@ -682,6 +682,12 @@
   #define CFG_TUD_MIDI2_PRODUCT_ID  "TinyUSB-MIDI2"
 #endif
 
+// String descriptor index for the Group Terminal Block (iBlockItem, Table 5-6).
+// 0 = no string descriptor (default, spec-allowed).
+#ifndef CFG_TUD_MIDI2_BLOCK_STRIDX
+  #define CFG_TUD_MIDI2_BLOCK_STRIDX 0
+#endif
+
 #ifndef CFG_TUD_VENDOR
   #define CFG_TUD_VENDOR          0
 #endif