ALSA: snd-usb-caiaq: Simplify single case to an 'if'

After removing code, only one case remains. So use an 'if' instead.

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