staging: comedi: mite: rename mite_ring member 'descriptors_dma_addr'

Rename this member of struct mite_ring to 'dma_addr' to help shorten the
long lines.

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>