ALSA: usb-audio: Fix incorrect size check for processing/extension units

The recently introduced unit descriptor validation had some bug for
processing and extension units, it counts a bControlSize byte twice so
it expected a bigger size than it should have been.  This seems
resulting in a probe error on a few devices.

Fix the calculation for proper checks of PU and EU.

Fixes: 57f87706 ("ALSA: usb-audio: More validations of descriptor units")
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20191114165613.7422-1-tiwai@suse.de


Signed-off-by: Takashi Iwai <tiwai@suse.de>