ALSA: hdspm - Use enums in hdspm_tco_ltc_frames()

This patch doesn't change functionality, it only improves readability
and fixes a copy&paste error in a comment.

Signed-off-by: Adrian Knoth <adi@drcomp.erfurt.thur.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>