staging: comedi: das08: Reduce conditional compilation

This code is used by some combination of the CONFIG_COMEDI_DAS08_CS,
CONFIG_COMEDI_DAS08_ISA, and CONFIG_COMEDI_DAS08_PCI and contains a lot
of conditional compilation.

Remove most of the conditional compilation, relying on the compiler to
optimize out unused static functions and data.  Use the '__maybe_unused'
tag for those functions that cause compiler warnings as a result of
this.

Also change the DO_COMEDI_DRIVER_REGISTER macro from a conditionally
defined macro to a manifest constant macro to allow it to be tested
outside the preprocessor (although this is not currently needed).

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Acked-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>