ALSA: usb-audio: Change in connectors control creation interface

Change build_connector_control() and get_connector_control_name()
so they take `struct usb_mixer_interface` as input argument instead
of `struct mixer_build`.

This is preliminary work to add support for connectors control
for UAC3 BADD devices. No functional change.

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