staging: comedi: s626: prefix function and variable names

Add the prefix `s626_` to all the static variables and functions that
don't already have it.
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>