Changed request/release of mite channels to use mite_channel struct

Changed request/release of mite channels to use mite_channel struct
pointers, instead of channel indices.  Changed many mite functions
to take mite_channel argument instead of a mite_struct and a
channel index.  Added mite_dma_descriptor_ring, since drivers
need to maintain the descriptors seperatly from a channel, since
the channel is reserved/released with each use but the
descriptor ring only changes when comedi's buffer is resized.  Made
ni_mio_common.c dynamically allocate mite channels for ai/ao
when needed, instead of permanently assigning mite channels to
subdevices.