Yet more preliminary work for the upcoming USB gadget support.

Now export the helpers to convert between legacy MIDI1 and UMP data
for handling the MIDI 1.0 USB interface.  The header file is moved to
include/sound.

The API functions are slightly changed, so that they can be used
without the direct access to snd_ump object.  The allocation is done
in ump.c itself as it's a simple kcalloc().

Link: https://lore.kernel.org/r/20230623075530.10976-1-tiwai@suse.deSigned-off-by: Takashi Iwai <tiwai@suse.de>