ALSA: virtuoso: use lower master clock with H6 daughterboard

Because of the unshielded connector cable, it is important to use as low
a master clock frequency as possible with the H6.

For double rate modes (64-96 kHz), the MCLK rate is unconditionally
lowered from 512x to 256x because the higher rate would not improve
anything.

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>