staging: comedi: s626: prefix macros in s626.h

Prefix the names of all the macros defined in "s626.h" with `S626_`.
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>