ALSA: snd-usb-caiaq: rename 'dev' to 'cdev'

This is needed in order to make the device namespace cleaner, and will
help when moving this driver over to dev_*() logging.

Signed-off-by: Daniel Mack <zonque@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>