staging: comedi: ni_pcidio: Grab MITE spinlock while preparing DMA.

When setting up the DMA for 'read' streaming acquisition command, grab
the MITE channel spinlock before preparing and arming the DMA.

Change inspired by ni_ai_setup_MITE_dma() in ni_mio_common.c.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>