staging: comedi: mite: remove mite member 'channel_allocated'

An allocated mite_channel will have its 'ring' member initialized
to point to the mite_ring that will be used for DMA. A non-allocated
mite_channel will have a 'ring' member set to NULL, either by a
channel release or due to the initial kzalloc of the 'mite' struct.

Refactor the code to use the mite_chan->ring to detect in a channel
is allocated and remove the unnecessary member.

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>