ALSA: usb-audio: fix memory leak on cval

With the current exit return path of the ctl_info allocation failure
cval is not being freed resulting in a memory leak. Fix this by kfree'ing
it on the return.

Detected by CoverityScan, CID#1466878 ("Resource Leak")

Fixes: 21e9b3e9 ("ALSA: usb-audio: fix uac control query argument")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>