It's not possible to put USAC into ADTS due to exceeding the
field size (4 bits) for ADTS profile. As such cases where
the frame starts with 0xfff shouldn't be checked.

This ensures the sample at https://crbug.com/396190942 is
properly detected as USAC content.

Signed-off-by: Dale Curtis <dalecurtis@chromium.org>