ALSA: usb-audio: Use __le16 for 16bit USB descriptor fields

Use proper notion for 16bit values for fixing the sparse warnings.

Fixes: f8ddb0fb

 ("ALSA: usb-audio: Define USB MIDI 2.0 specs")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202305260528.wcqjXso8-lkp@intel.com/
Closes: https://lore.kernel.org/oe-kbuild-all/202305270534.odwHL9F0-lkp@intel.com/
Link: https://lore.kernel.org/r/20230605144758.6677-1-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>