USB: mos7720: fix DMA to stack

The read_mos_reg function is called with stack-allocated buffers, which
must not be used 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>