ALSA: us122l: Drop mmap_count field

us122l.mmap_count field was used for counting the hwdep mmap opens and
syncing at disconnection.  But such a manual sync isn't needed, as the
refcount check is done in the ALSA core side.  So let's drop it.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Link: https://patch.msgid.link/20241113111042.15058-4-tiwai@suse.de