ALSA: usb-audio: Replace tasklet with work

ALSA: usb-audio: Replace tasklet with work

The tasklet is an old API that should be deprecated, usually can be
converted to another decent API.  In USB-audio driver, a tasklet is
still used in MIDI interface code for handling the output byte
stream.  It can be achieved gracefully with a work queued in the
high-prio system workqueue.

This patch replaces the tasklet usage in USB-audio driver with a
simple work.

Link: https://lore.kernel.org/r/20200903104131.21097-4-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>