ALSA: indigo-express: add missing 64KHz flags

Indigo-express cards also support 64KHz sampling rate: this patch adds
missing SNDRV_PCM_RATE_64000 flags.

Signed-off-by: Giuliano Pochini <pochini@shiny.it>
Signed-off-by: Takashi Iwai <tiwai@suse.de>