comedi: dt9812: fix DMA buffers on stack

In the Linux kernel, the following vulnerability has been resolved:

comedi: dt9812: fix DMA buffers on stack

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.