ALSA: mixart: Correct comment wrt obsoleted tasklet usage

The miXart driver has been already converted to use the threaded IRQ
instead of tasklet while there is a remaining comment still mentioning
a tasklet.  Update the comment appropriately.

Fixes: 8d3a8b5c ("ALSA: mixart: Use nonatomic PCM ops")
Link: https://lore.kernel.org/r/20200903104131.21097-12-tiwai@suse.de


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