staging: comedi: mite: rename 'struct mite_dma_descriptor_ring'

Rename this name to 'mite_ring' to help shorten the long lines.

In the ni_660x driver, also shorten the private data member 'mite_rings'
to simply 'ring'.

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>