staging: comedi: mite: make internal functions static

Declare some non-exported functions in "mite.c" `static` and remove
their declarations from "mite.h".

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>