staging: comedi: amplc_pc236: Remove most #if lines

Remove most of the #if IS_ENABLED(xxx) lines, but add extra if
(IS_ENABLED(xxx)) tests so the compiler can remove unreachable code.
The pci_driver stuff and device table is still conditionally compiled.

Also made pc236_find_pci() static as it was accidentally made non-static
by an earlier patch, so the compiler wouldn't remove it when it was
unreachable.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>