ALSA: virtio: drop an extaneous kernel-doc comment

ALSA: virtio: drop an extaneous kernel-doc comment

Drop a kernel-doc struct comment since the struct member was removed.
This eliminates a kernel-doc warning when make W=1 is used.

virtio_pcm.h:65: warning: Excess struct member 'msg_last_enqueued'
 description in 'virtio_pcm_substream'

Fixes: fe981e67568c ("ALSA: virtio: use ack callback")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Link: https://patch.msgid.link/20260216055956.2784399-1-rdunlap@infradead.org