staging: comedi: s626: move encoder functions part 1

Move some functions to help avoid the forward declaration of
`s626_enc_chan_info[]`.

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>