ALSA: timer: fix wrong comment to refer to 'SNDRV_TIMER_PSFLG_*'

ALSA timer core has a comment referring to 'SNDRV_MIXER_PSFLG_*' in
a definition of 'struct snd_timer_params' of UAPI header. I can see
this in initial state of ALSA timer core, at least in
'alsa-driver-0.4.0.tar.gz'.

This commit fixes the comment.

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>