staging: comedi: mite: introduce mite_free_dma_descs()

Introduce a helper function to handle the dma_free_coherent() of
the mite dma descriptors.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>