Use struct hdspm_ltc to query the LTC, using a mixer struct is just
plain wrong.

Due to the wrong struct, this ioctl was never working, so we're free to
fix it without breaking userspace compatibility.
Signed-off-by: Adrian Knoth <adi@drcomp.erfurt.thur.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>