ALSA: caiaq: input: Add error handling for unsupported input methods in `snd_usb_caiaq_input_init`

Smatch complains that:
snd_usb_caiaq_input_init() warn: missing error code 'ret'

This patch adds a new case to handle the situation where the
device does not support any input methods in the
`snd_usb_caiaq_input_init` function. It returns an `-EINVAL` error code
to indicate that no input methods are supported on the device.

Fixes: 523f1dce ("[ALSA] Add Native Instrument usb audio device support")
Signed-off-by: Ruliang Lin <u202112092@hust.edu.cn>
Reviewed-by: Dongliang Mu <dzm91@hust.edu.cn>
Acked-by: Daniel Mack <daniel@zonque.org>
Link: https://lore.kernel.org/r/20230504065054.3309-1-u202112092@hust.edu.cnSigned-off-by: Takashi Iwai <tiwai@suse.de>

sound/usb/caiaq/input.c

@@ -804,6 +804,7 @@ int snd_usb_caiaq_input_init(struct snd_usb_caiaqdev *cdev)
 
 	default:
 		/* no input methods supported on this device */
+		ret = -EINVAL;
 		goto exit_free_idev;
 	}