[ALSA] usbaudio - Coping with short replies in usbmixer

This patch makes sure that short USB replies are treated as an
error when requesting the value of a certain mixer control.

Signed-off-by: Thomas Reitmayr <thomas@devbase.at>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>