ALSA: usb-audio: Fix missing error check at mixer resolution test

A check of the return value from get_cur_mix_raw() is missing at the
resolution test code in get_min_max_with_quirks(), which may leave the
variable untouched, leading to a random uninitialized value, as
detected by syzkaller fuzzer.

Add the missing return error check for fixing that.

Reported-and-tested-by:  <syzbot+abe1ab7afc62c6bb6377@syzkaller.appspotmail.com>
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20191109181658.30368-1-tiwai@suse.de


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