ALSA: usb-audio: add front jack channel selector for EMU0204

Add support for front jack channel selector which is present on EMU0204.
It allows to get 4 channels out of this soundcard.

Tested-by: Yury Bushmelev <jay@jay-tech.ru>
Signed-off-by: Vasily Khoruzhick <anarsoul@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>