staging: comedi: mite: add mite_alloc() and mite_free()

Add `mite_alloc()` to allow drivers to allocate and initialize a `struct
mite_struct` dynamically and export it.  Add `mite_free()`, which is
currently just an inline wrapper for `kfree()`.

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>