ALSA: usb-audio: Fix parse of UAC2 Extension Units

Extension Unit (XU) is used to have a compatible layout with
Processing Unit (PU) on UAC1, and the usb-audio driver code assumed it
for parsing the descriptors.  Meanwhile, on UAC2, XU became slightly
incompatible with PU; namely, XU has a one-byte bmControls bitmap
while PU has two bytes bmControls bitmap.  This incompatibility
results in the read of a wrong address for the last iExtension field,
which ended up with an incorrect string for the mixer element name, as
recently reported for Focusrite Scarlett 18i20 device.

This patch corrects this misalignment by introducing a couple of new
macros and calling them depending on the descriptor type.

Fixes: 23caaf19

 ("ALSA: usb-mixer: Add support for Audio Class v2.0")
Reported-by: Stefan Sauer <ensonic@hora-obscura.de>
Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>