FROMLIST: 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")

am from commit 623760257b39 ("ALSA: usb-audio: fix memory leak on cval")
git://git.kernel.org/pub/scm/linux/kernel/git/tiwai/sound.git

 for-next

Change-Id: I500045647e3a926fb0d72620f9420656585a7171
Fixes: 21e9b3e931f7 ("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>