staging: comedi: mite: export mite_alloc_ring() and mite_free_ring()

The `mite_alloc_ring()` and `mite_free_ring()` static inline functions
in "mite.h" are reasonably large.  Transfer them to "mite.c" and export
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>