USB: ffs-test: fix length argument of out function call

commit eb9c5836 upstream.

The out functions should only handle actual available data instead of the complete buffer.
Otherwise for example the ep0_consume function will report ghost events since it tries to decode
the complete buffer - which may contain partly invalid data.

Signed-off-by: Matthias Fend <matthias.fend@wolfvision.net>
Acked-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>