ALSA: seq: Use flexible array for MIDI channels

ALSA: seq: Use flexible array for MIDI channels

Store MIDI channel entries in the MIDI channel set allocation instead
of allocating them separately.

This ties the channel array lifetime directly to the channel set, removes
a separate allocation failure path, and lets __counted_by() describe the
array bounds. Move the embedded emux channel set to the end of its
containing structure so it can carry the flexible array.

Assisted-by: Codex:GPT-5.5
Signed-off-by: Rosen Penev <rosenp@gmail.com>
Link: https://patch.msgid.link/20260511075447.445350-1-rosenp@gmail.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>