[ALSA] fix resource leak in usbmixer

Modules: USB generic driver

We may leak 'namelist' in sound/usb/usbmixer.c::parse_audio_selector_unit()

Signed-off-by: Jesper Juhl <jesper.juhl@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>