USB: mos7840: fix DMA to stack

Fix regression introduced by commit 0eafe4de1a ("USB: serial: mos7840:
add support for MCS7810 devices") which used stack-allocated buffers for
control messages.

Cc: stable@vger.kernel.org
Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>