[ALSA] fix usbmixer double kfree

Modules: USB generic driver

snd_ctl_add() kfree's the kcontrol already if we fail there,
so this driver is currently doing a double kfree.

Coverity bug #959

Signed-off-by: Dave Jones <davej@redhat.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>