ALSA: usb-audio: Always create the interrupt pipe for the mixer

An UAC3 BADD device may also include an interrupt status pipe
to report changes on the HEADSET ADAPTER terminals. The creation
of the status pipe is dependent on the device reporting that it
has it.

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