staging: comedi: mite: don't export internal functions

The functions that get the number of bytes written to and read from
memory are only used internally by the mite driver when syncing the
dma channel. Make them static and remove the exports.

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>