[ALSA] Fix sleep in atomic in virmidi driver

Modules: ALSA sequencer

 Debug: sleeping function called from invalid context at /usr/src/linux/include/linux/rwsem.h:43
 in_atomic():1, irqs_disabled():0
  [<f999d15e>] snd_seq_deliver_event+0xb4/0x1a8 [snd_seq]
  [<f999d2be>] snd_seq_kernel_client_dispatch+0x6c/0x7c [snd_seq]
  [<f93321fc>] snd_virmidi_output_trigger+0xca/0xe5 [snd_seq_virmidi]

Signed-off-by: Takashi Iwai <tiwai@suse.de>