ALSA: snd_usb_caiaq: fix legacy input streaming

Seems that nobody recently tried the input on the very first supported
sound card model, RK2. This patch fixes the byte offset to make it
running again.

Signed-off-by: Daniel Mack <daniel@caiaq.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>