ALSA: usb-audio: Add insertion control for UAC3 BADD

The HEADSET ADAPTER profile for BADD devices is meant to support
Insertion Control for the Input and Output Terminals of the headset.

This patch defines the BADD inferred input and output terminals and
builds the connector controls.

Signed-off-by: Jorge Sanjuan <jorge.sanjuan@codethink.co.uk>
Signed-off-by: Takashi Iwai <tiwai@suse.de>