ALSA: snd-usb-caiaq: Do not expose hardware input mode 0 of A4DJ

In the context of the Audio 4 DJ (when compared to Audio 8 DJ), hardware
input mode 0 is not used. Expose modes 1 (line) and 2 (phono) to the user
as modes 0 and 1 respectively.

Signed-off-by: Mark Hills <mark@pogo.org.uk>
Acked-by: Daniel Mack <daniel@caiaq.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>