staging: comedi: mite: remove list of devices

All the drivers that use the "mite" module now allocate a `struct
mite_struct` dynamically instead of searching the `mite_devices` list
populated during initialization of the "mite" module.

Remove the list of devices and the function that prints available NI
devices on the list (`mite_list_devices()`).  The list node and `used`
members in `struct mite_struct` are now redundant, so remove them.

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