comedi: dt9812: fix DMA buffers on stack

commit 536de747 upstream.

USB transfer buffers are typically mapped for DMA and must not be
allocated on the stack or transfers will fail.

Allocate proper transfer buffers in the various command helpers and
return an error on short transfers instead of acting on random stack
data.

Note that this also fixes a stack info leak on systems where DMA is not
used as 32 bytes are always sent to the device regardless of how short
the command is.

Fixes: 63274cd7

 ("Staging: comedi: add usb dt9812 driver")
Cc: stable@vger.kernel.org      # 2.6.29
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Johan Hovold <johan@kernel.org>
Link: https://lore.kernel.org/r/20211027093529.30896-3-johan@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>