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, 3 insertions, 0 deletions

diff --git a/test/unit-test/test/device/midi2/test_midi2_device.c b/test/unit-test/test/device/midi2/test_midi2_device.c
index 9d716d93b..1314c2585 100644
--- a/test/unit-test/test/device/midi2/test_midi2_device.c
+++ b/test/unit-test/test/device/midi2/test_midi2_device.c
@@ -147,6 +147,9 @@ void test_midi2_descriptor_bytes(void) {
   TEST_ASSERT_EQUAL(MIDI_CS_INTERFACE_HEADER, desc[ms2_offset + 2]);
   TEST_ASSERT_EQUAL(0x00, desc[ms2_offset + 3]);
   TEST_ASSERT_EQUAL(0x02, desc[ms2_offset + 4]);
+  // USB-MIDI 2.0 Table 5-2: wTotalLength shall match bLength (= 0x0007)
+  TEST_ASSERT_EQUAL(0x07, desc[ms2_offset + 5]);
+  TEST_ASSERT_EQUAL(0x00, desc[ms2_offset + 6]);
 }
 
 void test_midi2_descriptor_alt1_cs_endpoint_subtype(void) {