ALSA: usb-audio: Set the Control Selector to SU_SELECTOR_CONTROL for UAC2

Specified in section 5.2.5.6.1 of the USB Audio Class 2.0 definition.

Solves the following error for C-Media 6632A (Asus Xonar U7):
[ 8219.676164] cannot get ctl value: req = 0x81, wValue = 0x0, wIndex = 0x1400, type = 3

Signed-off-by: Johan Rastén <johan@oljud.se>
Signed-off-by: Takashi Iwai <tiwai@suse.de>